feat(maintenance): add a maintenance notification during backend migration

frontend/src/lib/api/client.ts

@@ -18,6 +18,15 @@
  */
 
 import { getCsrfHeaders } from './csrf';
+import { updateFromHeader } from '$lib/stores/serverStatus.svelte';
+
+/**
+ * Name of the response header the server stamps while a
+ * maintenance event is live. Case-insensitive on the wire — the
+ * Fetch API's `Headers.get` matches irrespective of case, so this
+ * constant matches whatever axum emits.
+ */
+const SERVER_STATUS_HEADER = 'x-server-status';
 
 const REFRESH_ENDPOINT = '/api/auth/refresh';
 
@@ -93,6 +102,18 @@ export function createApiFetch(deps: ApiClientDeps): FetchFn {
 	const apiFetch: FetchFn = async (input, init) => {
 		const origin = deps.origin ?? globalThis.location?.origin ?? 'http://localhost';
 		const response = await rawFetch(input, init);
+		// Server-status header piggyback — the server stamps
+		// `x-server-status` on every response while a maintenance
+		// event is in progress (see middleware::server_status). Read
+		// it and update the reactive store; the AppShell banner
+		// subscribes and shows/hides itself. Absent header = nothing
+		// happening; the update fn resets the store to default in
+		// that case so a lingering banner disappears.
+		//
+		// Runs on EVERY response including a 401 (below) so a session
+		// refresh doesn't accidentally clear a live banner.
+		updateFromHeader(response.headers.get(SERVER_STATUS_HEADER));
+
 		if (response.status !== 401) return response;
 
 		const urlStr = urlString(input as RequestInfo | URL);
@@ -104,7 +125,9 @@ export function createApiFetch(deps: ApiClientDeps): FetchFn {
 			onSessionExpired();
 			throw new Error('Session expired');
 		}
-		return rawFetch(input, init);
+		const retryResponse = await rawFetch(input, init);
+		updateFromHeader(retryResponse.headers.get(SERVER_STATUS_HEADER));
+		return retryResponse;
 	};
 
 	return apiFetch;

frontend/src/lib/components/AppShell.svelte

@@ -11,10 +11,12 @@
 	import type { FileItem, FolderItem, ItemType } from '$lib/api/types';
 	import { lazyComponent } from '$lib/composables/lazyComponent.svelte';
 	import DrivePicker from '$lib/components/DrivePicker.svelte';
+	import ReadOnlyBanner from '$lib/components/ReadOnlyBanner.svelte';
 	import Icon from '$lib/icons/Icon.svelte';
 	import { dateTimeFormatFor, iconNameFromClass } from '$lib/utils/display';
 	import { userInitials, avatarColorIndex } from '$lib/utils/avatar';
 	import { i18n, LANGUAGES, setLocale, t, type Locale } from '$lib/i18n/index.svelte';
+	import { serverStatus } from '$lib/stores/serverStatus.svelte';
 	import { apiFetch } from '$lib/api/client';
 	import { dialogs } from '$lib/stores/dialogs.svelte';
 	import { files as filesStore } from '$lib/stores/files.svelte';
@@ -1025,6 +1027,21 @@
 	</div>
 
 	<div class="content-area">
+		<!-- Server-wide maintenance banner. Fed by the
+		     `x-server-status` header read on every API response by
+		     `apiFetch` — no polling. Shows for every logged-in user
+		     while a storage migration is running so they know why
+		     writes are being refused, with live progress if
+		     available. Disappears automatically on the next API
+		     round-trip after the server clears the flag.
+
+		     Reuses `ReadOnlyBanner` (same component that renders a
+		     drive-frozen notice) with `variant="maintenance"` so the
+		     two banners are visually indistinguishable — just
+		     different copy. -->
+		{#if serverStatus().readonly}
+			<ReadOnlyBanner variant="maintenance" progress={serverStatus().migration} />
+		{/if}
 		{@render children()}
 	</div>
 </div>

frontend/src/lib/components/ReadOnlyBanner.svelte

@@ -1,6 +1,8 @@
 <script lang="ts">
 	/**
-	 * Read-only drive banner.
+	 * Read-only banner — one component, two variants.
+	 *
+	 * ## `variant="drive"` (default) — drive-scoped freeze
 	 *
 	 * Rendered at the top of any page whose content lives in (or is scoped
 	 * to) a drive whose `policies.read_only === true`. Members see the
@@ -19,33 +21,60 @@
 	 *   - `routes/files/[...path]/+page.svelte` — shown when the current
 	 *     folder's owning drive is frozen (parent looks up drive via
 	 *     `drives.findByRootFolderId`/`findById`).
-	 *   - Future: `/photos`, `/music`, and any other drive-scoped views.
+	 *
+	 * ## `variant="maintenance"` — server-wide freeze
+	 *
+	 * Rendered inside `AppShell` above `{children}` when the
+	 * `x-server-status` header (see `middleware::server_status`) says
+	 * the whole server is in read-only mode — typically during a
+	 * storage-backend migration. Optional `progress` lets the banner
+	 * show target + percentage.
+	 *
+	 * Shape / accent is identical between both variants — the design
+	 * system reads them as the same family. Only the copy differs.
 	 */
 	import { t } from '$lib/i18n/index.svelte';
 	import Icon from '$lib/icons/Icon.svelte';
 
-	interface Props {
-		/** Drive-name shown in the body so members know which drive the
-		 *  freeze applies to. Optional — omit on pages where the drive is
-		 *  implicit from context (e.g. the drive's own config page). */
-		driveName?: string;
+	interface Progress {
+		target: string;
+		migrated: number;
+		total: number;
+		percent: number;
 	}
 
-	let { driveName }: Props = $props();
+	interface Props {
+		/**
+		 * `"drive"` — a specific drive is frozen (default; back-compat
+		 * with pre-migration call sites). `"maintenance"` — the whole
+		 * server is in read-only mode.
+		 */
+		variant?: 'drive' | 'maintenance';
+		/** Drive-name shown in the body (variant="drive" only). */
+		driveName?: string;
+		/** Migration progress (variant="maintenance" only). */
+		progress?: Progress;
+	}
+
+	let { variant = 'drive', driveName, progress }: Props = $props();
 </script>
 
 <div
 	class="read-only-banner"
 	role="region"
-	aria-label={t('drive.read_only_banner.aria', 'This drive is read-only')}
-	data-testid="read-only-banner"
+	aria-label={variant === 'maintenance'
+		? t('server_status.readonly_banner_aria', 'Server maintenance in progress')
+		: t('drive.read_only_banner.aria', 'This drive is read-only')}
+	data-testid={variant === 'maintenance' ? 'server-status-banner' : 'read-only-banner'}
 >
 	<div class="read-only-banner__icon" aria-hidden="true">
 		<Icon name="lock" />
 	</div>
 	<div class="read-only-banner__body">
 		<strong>
-			{#if driveName}
+			{#if variant === 'maintenance'}
+				{t('server_status.readonly_title', 'Server maintenance in progress')}
+			{:else if driveName}
 				{t(
 					'drive.read_only_banner.title_named',
 					{ name: driveName },
@@ -56,10 +85,30 @@
 			{/if}
 		</strong>
 		<span>
-			{t(
-				'drive.read_only_banner.body',
-				'Uploads, edits, deletes, renames, sharing and membership changes are refused. Reads and downloads keep working. Contact an administrator to un-freeze the drive.'
-			)}
+			{#if variant === 'maintenance'}
+				{#if progress}
+					{t(
+						'server_status.readonly_progress',
+						{
+							target: progress.target,
+							migrated: progress.migrated,
+							total: progress.total,
+							percent: progress.percent
+						},
+						'Migrating storage to `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blobs). Uploads, renames, deletes, and shares are refused; reads and downloads work as normal.'
+					)}
+				{:else}
+					{t(
+						'server_status.readonly_body',
+						'Uploads, renames, deletes, and shares are refused temporarily. Reads and downloads work as normal.'
+					)}
+				{/if}
+			{:else}
+				{t(
+					'drive.read_only_banner.body',
+					'Uploads, edits, deletes, renames, sharing and membership changes are refused. Reads and downloads keep working. Contact an administrator to un-freeze the drive.'
+				)}
+			{/if}
 		</span>
 	</div>
 </div>

frontend/src/lib/stores/serverStatus.svelte.ts

@@ -0,0 +1,67 @@
+/**
+ * Reactive server-status store.
+ *
+ * Populated by the `apiFetch` wrapper, which reads the
+ * `x-server-status` header off every API response and calls
+ * `updateFromHeader(...)`. When no migration is running the header
+ * is absent and the store stays at its default (readonly=false, no
+ * migration info). See `middleware::server_status` on the server
+ * for the header spec.
+ *
+ * The AppShell subscribes to this store to show/hide the
+ * maintenance banner without polling — the state travels back to
+ * the client on the piggyback of whatever API request the user was
+ * making anyway. Zero extra network cost.
+ */
+
+/**
+ * JSON shape emitted in the `x-server-status` header. Optional
+ * `migration` field is present only while a migration is running.
+ */
+export interface ServerStatus {
+	readonly: boolean;
+	migration?: {
+		target: string;
+		migrated: number;
+		total: number;
+		percent: number;
+	};
+}
+
+const DEFAULT: ServerStatus = { readonly: false };
+
+// Rune-based reactive state — `$state` in a `.svelte.ts` module.
+let current = $state<ServerStatus>(DEFAULT);
+
+/** Current server status. Reactively updates when apiFetch sees a new header. */
+export function serverStatus(): ServerStatus {
+	return current;
+}
+
+/**
+ * Parse the raw header value and update the store. Silently
+ * tolerates a missing header (resets to default: nothing to
+ * broadcast means nothing wrong) and a malformed one (keeps the
+ * previous value rather than surface a parse error to users).
+ *
+ * Called by `apiFetch` after every response — see `client.ts`.
+ */
+export function updateFromHeader(rawHeader: string | null): void {
+	if (rawHeader == null) {
+		// No header on this response = server not in maintenance
+		// mode = reset the store to the default so any lingering
+		// banner disappears. Cheap idempotent write.
+		if (current.readonly || current.migration) current = DEFAULT;
+		return;
+	}
+	try {
+		const parsed = JSON.parse(rawHeader) as ServerStatus;
+		// Basic shape validation — server should never send a
+		// missing `readonly`, but be defensive.
+		if (typeof parsed.readonly === 'boolean') {
+			current = parsed;
+		}
+	} catch {
+		// Malformed header — keep previous state rather than churn.
+	}
+}

frontend/static/locales/ar.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "صيانة الخادم جارية",
+    "readonly_title": "صيانة الخادم جارية",
+    "readonly_progress": "جارٍ نقل التخزين إلى `{{target}}` — {{percent}}٪ ({{migrated}} / {{total}} كتلة). الرفع وإعادة التسمية والحذف والمشاركة مرفوضة؛ القراءة والتنزيل تعملان بشكل طبيعي.",
+    "readonly_body": "الرفع وإعادة التسمية والحذف والمشاركة مرفوضة مؤقتًا. القراءة والتنزيل يعملان بشكل طبيعي."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "نظام تخزين سحابي بسيط"

frontend/static/locales/de.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Serverwartung läuft",
+    "readonly_title": "Serverwartung läuft",
+    "readonly_progress": "Speicher wird auf `{{target}}` migriert — {{percent}} % ({{migrated}} / {{total}} Blöcke). Uploads, Umbenennungen, Löschungen und Freigaben werden abgelehnt; Lesen und Herunterladen funktionieren normal.",
+    "readonly_body": "Uploads, Umbenennungen, Löschungen und Freigaben werden vorübergehend abgelehnt. Lesen und Herunterladen funktionieren normal."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Minimalistisches Cloud-Speichersystem"

frontend/static/locales/en.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Server maintenance in progress",
+    "readonly_title": "Server maintenance in progress",
+    "readonly_progress": "Migrating storage to `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blobs). Uploads, renames, deletes, and shares are refused; reads and downloads work as normal.",
+    "readonly_body": "Uploads, renames, deletes, and shares are refused temporarily. Reads and downloads work as normal."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Minimalist cloud storage system"

frontend/static/locales/es.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Mantenimiento del servidor en curso",
+    "readonly_title": "Mantenimiento del servidor en curso",
+    "readonly_progress": "Migrando el almacenamiento a `{{target}}` — {{percent}} % ({{migrated}} / {{total}} bloques). Las subidas, renombres, eliminaciones y comparticiones se rechazan; las lecturas y descargas funcionan con normalidad.",
+    "readonly_body": "Las subidas, renombres, eliminaciones y comparticiones se rechazan temporalmente. Las lecturas y descargas funcionan con normalidad."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Sistema de almacenamiento en la nube minimalista"

frontend/static/locales/fa.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "نگهداری سرور در حال انجام است",
+    "readonly_title": "نگهداری سرور در حال انجام است",
+    "readonly_progress": "در حال انتقال حافظه به `{{target}}` — {{percent}}٪ ({{migrated}} / {{total}} بلاک). آپلود، تغییر نام، حذف و اشتراک‌گذاری رد می‌شوند؛ خواندن و دانلود عادی کار می‌کنند.",
+    "readonly_body": "آپلود، تغییر نام، حذف و اشتراک‌گذاری موقتاً رد می‌شوند. خواندن و دانلود عادی کار می‌کنند."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "سیستم ذخیره‌سازی ابری ساده‌گرا"

frontend/static/locales/fr.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Maintenance du serveur en cours",
+    "readonly_title": "Maintenance du serveur en cours",
+    "readonly_progress": "Migration du stockage vers `{{target}}` — {{percent}} % ({{migrated}} / {{total}} blocs). Les téléversements, renommages, suppressions et partages sont refusés ; la lecture et le téléchargement continuent normalement.",
+    "readonly_body": "Les téléversements, renommages, suppressions et partages sont temporairement refusés. La lecture et le téléchargement fonctionnent normalement."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Système de stockage cloud minimaliste"

frontend/static/locales/hi.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "सर्वर रखरखाव प्रगति पर है",
+    "readonly_title": "सर्वर रखरखाव प्रगति पर है",
+    "readonly_progress": "स्टोरेज को `{{target}}` पर माइग्रेट किया जा रहा है — {{percent}}% ({{migrated}} / {{total}} ब्लॉब्स)। अपलोड, नाम बदलना, हटाना और साझा करना अस्वीकृत हैं; पढ़ना और डाउनलोड सामान्य रूप से काम करते हैं।",
+    "readonly_body": "अपलोड, नाम बदलना, हटाना और साझा करना अस्थायी रूप से अस्वीकृत हैं। पढ़ना और डाउनलोड सामान्य रूप से काम करते हैं।"
+  },
   "app": {
     "title": "OxiCloud",
     "description": "न्यूनतम क्लाउड स्टोरेज सिस्टम"

frontend/static/locales/it.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Manutenzione del server in corso",
+    "readonly_title": "Manutenzione del server in corso",
+    "readonly_progress": "Migrazione dello storage verso `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blob). Caricamenti, rinomine, eliminazioni e condivisioni sono rifiutati; le letture e i download funzionano normalmente.",
+    "readonly_body": "Caricamenti, rinomine, eliminazioni e condivisioni sono temporaneamente rifiutati. Le letture e i download funzionano normalmente."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Sistema di archiviazione cloud minimalista"

frontend/static/locales/ja.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "サーバーメンテナンス中",
+    "readonly_title": "サーバーメンテナンス中",
+    "readonly_progress": "ストレージを `{{target}}` に移行中 — {{percent}}%（{{migrated}} / {{total}} ブロブ）。アップロード、名前変更、削除、共有は拒否されます。読み取りとダウンロードは通常どおり動作します。",
+    "readonly_body": "アップロード、名前変更、削除、共有は一時的に拒否されます。読み取りとダウンロードは通常どおり動作します。"
+  },
   "app": {
     "title": "OxiCloud",
     "description": "ミニマリストクラウドストレージシステム"

frontend/static/locales/ko.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "서버 유지 관리 진행 중",
+    "readonly_title": "서버 유지 관리 진행 중",
+    "readonly_progress": "저장소를 `{{target}}`(으)로 마이그레이션 중 — {{percent}}% ({{migrated}} / {{total}} 블롭). 업로드, 이름 변경, 삭제 및 공유가 거부됩니다. 읽기 및 다운로드는 정상적으로 작동합니다.",
+    "readonly_body": "업로드, 이름 변경, 삭제 및 공유가 일시적으로 거부됩니다. 읽기 및 다운로드는 정상적으로 작동합니다."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "미니멀리스트 클라우드 스토리지 시스템"

frontend/static/locales/nl.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Serveronderhoud bezig",
+    "readonly_title": "Serveronderhoud bezig",
+    "readonly_progress": "Opslag wordt gemigreerd naar `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blobs). Uploads, hernoemingen, verwijderingen en delen worden geweigerd; lezen en downloaden werken normaal.",
+    "readonly_body": "Uploads, hernoemingen, verwijderingen en delen worden tijdelijk geweigerd. Lezen en downloaden werken normaal."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Minimalistisch cloudopslagsysteem"

frontend/static/locales/pl.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Trwa konserwacja serwera",
+    "readonly_title": "Trwa konserwacja serwera",
+    "readonly_progress": "Migracja pamięci do `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blobów). Przesyłanie, zmiana nazwy, usuwanie i udostępnianie są odrzucane; odczyt i pobieranie działają normalnie.",
+    "readonly_body": "Przesyłanie, zmiana nazwy, usuwanie i udostępnianie są tymczasowo odrzucane. Odczyt i pobieranie działają normalnie."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Minimalistyczny cloud storage"

frontend/static/locales/pt.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Manutenção do servidor em curso",
+    "readonly_title": "Manutenção do servidor em curso",
+    "readonly_progress": "A migrar o armazenamento para `{{target}}` — {{percent}}% ({{migrated}} / {{total}} blobs). Envios, renomeações, eliminações e partilhas são recusados; leituras e transferências funcionam normalmente.",
+    "readonly_body": "Envios, renomeações, eliminações e partilhas são temporariamente recusados. Leituras e transferências funcionam normalmente."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Sistema de armazenamento em nuvem minimalista"

frontend/static/locales/ru.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "Идёт обслуживание сервера",
+    "readonly_title": "Идёт обслуживание сервера",
+    "readonly_progress": "Миграция хранилища на `{{target}}` — {{percent}}% ({{migrated}} / {{total}} блобов). Загрузки, переименования, удаления и общий доступ отклоняются; чтение и скачивание работают как обычно.",
+    "readonly_body": "Загрузки, переименования, удаления и общий доступ временно отклоняются. Чтение и скачивание работают как обычно."
+  },
   "app": {
     "title": "OxiCloud",
     "description": "Минималистичная система облачного хранения"

frontend/static/locales/zh-TW.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "伺服器維護進行中",
+    "readonly_title": "伺服器維護進行中",
+    "readonly_progress": "正在將儲存遷移至 `{{target}}` — {{percent}}%（{{migrated}} / {{total}} 個 blob）。上傳、重新命名、刪除和分享會被拒絕；讀取和下載正常運作。",
+    "readonly_body": "上傳、重新命名、刪除和分享暫時被拒絕。讀取和下載正常運作。"
+  },
   "app": {
     "title": "OxiCloud",
     "description": "極簡雲端儲存系統"

frontend/static/locales/zh.json

@@ -38,6 +38,12 @@
       }
     }
   },
+  "server_status": {
+    "readonly_banner_aria": "服务器维护进行中",
+    "readonly_title": "服务器维护进行中",
+    "readonly_progress": "正在将存储迁移到 `{{target}}` — {{percent}}%（{{migrated}} / {{total}} 个 blob）。上传、重命名、删除和共享被拒绝；读取和下载正常工作。",
+    "readonly_body": "上传、重命名、删除和共享暂时被拒绝。读取和下载正常工作。"
+  },
   "app": {
     "title": "OxiCloud",
     "description": "极简云存储系统"

src/common/di.rs

@@ -2028,6 +2028,7 @@ impl AppServiceFactory {
                 crate::infrastructure::services::webdav_dead_property_store::create_dead_property_store(pool.clone()),
             authorization: authorization.clone(),
             migration_readonly: migration_readonly.clone(),
+            migration_progress: Arc::new(std::sync::RwLock::new(None)),
             drive_repo: drive_repo.clone(),
             drive_management_service: Arc::new(
                 crate::application::services::drive_management_service::DriveManagementService::new(
@@ -2254,6 +2255,7 @@ impl AppServiceFactory {
                     self.storage_path.clone(),
                     app_state.migration_readonly.clone(),
                     app_state.core.blob_backend_hot_swap.clone(),
+                    app_state.migration_progress.clone(),
                 ),
             )
             .register_recoverable_job(&app_state.core.job_registry, &job_store_provider_dyn)
@@ -2785,6 +2787,14 @@ pub struct AppState {
     /// memory in sync. See `docs/plan/storage-multi-entry.md`
     /// §"Read-only mode".
     pub migration_readonly: Arc<std::sync::atomic::AtomicBool>,
+    /// Live progress snapshot for the storage-migration handler.
+    /// `Some(_)` while a migration is running; `None` otherwise.
+    /// Updated by the handler on every batch checkpoint (cheap
+    /// in-memory write, no DB read on the request path). The
+    /// server-status header middleware reads it to inform every
+    /// user's session banner about maintenance progress without
+    /// polling. See `MigrationProgress` for the field shape.
+    pub migration_progress: Arc<std::sync::RwLock<Option<crate::common::migration_progress::MigrationProgress>>>,
     /// Drive entity repository — `GET /api/drives`, the personal-drive
     /// lifecycle hook, and (post-D2) shared-drive creation flow all read
     /// through this. Backing table is `storage.drives`; membership is

src/common/migration_progress.rs

@@ -0,0 +1,66 @@
+//! Shared in-memory snapshot of the running storage-migration.
+//!
+//! Consumed by the server-status middleware to build the
+//! `X-Server-Status` response header on every authenticated
+//! request. That header lets every logged-in user's session banner
+//! show current maintenance progress without any polling —
+//! the state travels back on the piggyback of whatever API call
+//! the user was going to make anyway.
+//!
+//! Written by the migration handler on each batch checkpoint (a
+//! cheap `RwLock::write` + a small struct copy — no DB access on
+//! the request path). Cleared on `RunOutcome::Completed` /
+//! `Paused` / `Failed`. `None` means "no migration is running";
+//! middleware omits the header entirely in that case.
+
+use serde::Serialize;
+
+/// One snapshot of a running migration. Every field is a scalar so
+/// the whole struct copies cheaply under the `RwLock::write` guard.
+#[derive(Debug, Clone, Serialize)]
+pub struct MigrationProgress {
+    /// The entry name blobs are being copied INTO. Used by the
+    /// user-facing banner text so admins/users know what the
+    /// server is switching to.
+    pub target_name: String,
+    /// Blobs migrated so far this run. Starts at 0 on a Fresh
+    /// open; on Resume the checkpointed value is loaded from the
+    /// run row's `stats.scanned_count`.
+    pub migrated_blobs: u64,
+    /// Total blobs in the current DB snapshot. Captured once at
+    /// run start via `SELECT COUNT(*) FROM storage.blobs`. Doesn't
+    /// change during the run — new uploads are refused while
+    /// read-only is engaged, so the denominator stays honest.
+    pub total_blobs: u64,
+    /// Convenience: `migrated_blobs * 100 / total_blobs`, clamped
+    /// to 0..=100. Middleware could compute it but it's tiny and
+    /// makes the JSON payload obvious.
+    pub percent: u8,
+}
+
+impl MigrationProgress {
+    pub fn new(target_name: String, total_blobs: u64) -> Self {
+        Self {
+            target_name,
+            migrated_blobs: 0,
+            total_blobs,
+            percent: 0,
+        }
+    }
+
+    /// Update the counter + recompute `percent`. Called by the
+    /// migration handler after each batch checkpoint.
+    pub fn bump(&mut self, migrated_delta: u64) {
+        self.migrated_blobs = self.migrated_blobs.saturating_add(migrated_delta);
+        self.recompute_percent();
+    }
+
+    fn recompute_percent(&mut self) {
+        self.percent = if self.total_blobs == 0 {
+            0
+        } else {
+            ((self.migrated_blobs.min(self.total_blobs) as u128 * 100) / self.total_blobs as u128)
+                as u8
+        };
+    }
+}

src/common/mod.rs

@@ -3,6 +3,7 @@ pub mod di;
 pub mod errors;
 pub mod fmt;
 pub mod locale;
+pub mod migration_progress;
 pub mod mime_detect;
 pub mod runtime;
 pub mod stubs;

src/infrastructure/services/storage_migration_service.rs

@@ -121,6 +121,12 @@ pub struct StorageMigrationService {
     /// delegates through.
     blob_backend_hot_swap:
         Arc<crate::infrastructure::services::swappable_blob_backend::SwappableBlobBackend>,
+    /// Shared in-memory progress snapshot. `Some(_)` during a
+    /// running/paused migration, `None` otherwise. Read by the
+    /// server-status header middleware to broadcast maintenance
+    /// state to every user's session without polling.
+    migration_progress:
+        Arc<std::sync::RwLock<Option<crate::common::migration_progress::MigrationProgress>>>,
 }
 
 impl StorageMigrationService {
@@ -135,6 +141,9 @@ impl StorageMigrationService {
         blob_backend_hot_swap: Arc<
             crate::infrastructure::services::swappable_blob_backend::SwappableBlobBackend,
         >,
+        migration_progress: Arc<
+            std::sync::RwLock<Option<crate::common::migration_progress::MigrationProgress>>,
+        >,
     ) -> Self {
         Self {
             pool,
@@ -144,6 +153,7 @@ impl StorageMigrationService {
             storage_path_fallback,
             migration_readonly,
             blob_backend_hot_swap,
+            migration_progress,
         }
     }
 
@@ -384,6 +394,27 @@ impl RecoverableJobHandler for StorageMigrationService {
              cutover hot-swap completes"
         );
 
+        // Seed the shared progress snapshot for the header
+        // middleware. Total blob count is a one-shot SELECT COUNT(*)
+        // — best-effort; if it fails we still push a snapshot with
+        // total=0 so the banner at least shows *something* is
+        // happening.
+        let total_blobs: u64 = sqlx::query_scalar::<_, i64>("SELECT COUNT(*) FROM storage.blobs")
+            .fetch_one(self.pool.as_ref())
+            .await
+            .map(|n| n.max(0) as u64)
+            .unwrap_or(0);
+        {
+            let mut guard = self
+                .migration_progress
+                .write()
+                .unwrap_or_else(std::sync::PoisonError::into_inner);
+            *guard = Some(crate::common::migration_progress::MigrationProgress::new(
+                target_name.clone(),
+                total_blobs,
+            ));
+        }
+
         let source_kind = self.source.backend_type();
         let target_kind = target.backend_type();
         tracing::info!(
@@ -613,6 +644,19 @@ impl RecoverableJobHandler for StorageMigrationService {
                     message: format!("checkpoint: {e}"),
                 };
             }
+            // Bump the shared progress snapshot so the server-status
+            // header middleware surfaces fresh numbers on every
+            // user's next API call. Guard is held only for a struct
+            // update — microseconds.
+            {
+                let mut guard = self
+                    .migration_progress
+                    .write()
+                    .unwrap_or_else(std::sync::PoisonError::into_inner);
+                if let Some(progress) = guard.as_mut() {
+                    progress.bump(batch_len);
+                }
+            }
 
             if (rows.len() as i64) < BATCH_SIZE {
                 return self
@@ -703,6 +747,16 @@ impl StorageMigrationService {
             .await
             .is_ok();
         self.migration_readonly.store(false, Ordering::Relaxed);
+        // Clear the shared progress snapshot so the server-status
+        // header stops emitting on subsequent requests. Guard held
+        // only for the assignment.
+        {
+            let mut guard = self
+                .migration_progress
+                .write()
+                .unwrap_or_else(std::sync::PoisonError::into_inner);
+            *guard = None;
+        }
 
         if !readonly_persisted {
             tracing::warn!(

src/interfaces/api/routes.rs

@@ -687,13 +687,24 @@ pub fn create_api_routes(app_state: &Arc<AppState>) -> Router<Arc<AppState>> {
     // them on every overlapping request.
     router = router.route("/{*rest}", any(api_not_found));
 
-    // No per-router layers: the global `TraceLayer` + request-id stack in
-    // `main.rs` wraps the whole app (this `/api` router is nested into it),
-    // so a second `TraceLayer` here just double-wrapped every `/api`
-    // request in a redundant span + response-future poll (benches/ROUND13.md
-    // §H1). Compression is likewise the global layer's job — re-applying it
-    // here (no predicate) would compress media downloads, burning CPU for
-    // ~0 gain and stripping `Content-Length`.
+    // Server-status header. Stamps `X-Server-Status` on every
+    // response so the frontend's fetch wrapper can update a
+    // reactive store — banner shows/hides without polling.
+    // Sub-nanosecond on the hot path (single atomic load), a few
+    // µs on the cold path (only during a running migration). See
+    // `middleware::server_status`.
+    let router = router.layer(axum::middleware::from_fn_with_state(
+        app_state.clone(),
+        crate::interfaces::middleware::server_status::server_status_middleware,
+    ));
+
+    // No per-router layers beyond that: the global `TraceLayer` + request-id
+    // stack in `main.rs` wraps the whole app (this `/api` router is nested
+    // into it), so a second `TraceLayer` here just double-wrapped every
+    // `/api` request in a redundant span + response-future poll
+    // (benches/ROUND13.md §H1). Compression is likewise the global layer's
+    // job — re-applying it here (no predicate) would compress media
+    // downloads, burning CPU for ~0 gain and stripping `Content-Length`.
     router
 }
 

src/interfaces/middleware/mod.rs

@@ -3,6 +3,7 @@ pub mod auth;
 pub mod csrf;
 pub mod locale;
 pub mod rate_limit;
+pub mod server_status;
 pub mod trace_span;
 pub mod trusted_proxy;
 pub mod user;

src/interfaces/middleware/server_status.rs

@@ -0,0 +1,119 @@
+//! Middleware that stamps `X-Server-Status` on every response.
+//!
+//! Consumed by the frontend `apiFetch` wrapper — every API round-trip
+//! carries the current server maintenance state back to the client
+//! (no polling, no dedicated endpoint). The banner in the app shell
+//! subscribes to a store the wrapper updates and shows/hides itself
+//! reactively. See `docs/plan/storage-multi-entry.md` §"Read-only mode"
+//! for the broader design.
+//!
+//! ## Cost model
+//!
+//! On the *hot path* (no migration running — the ~100% case in normal
+//! operation) this middleware does:
+//!   1. one `AtomicBool::load(Relaxed)` — sub-nanosecond;
+//!   2. an early return when `false`.
+//!
+//! No allocation, no lock, no formatting. Adds no measurable latency
+//! at any user count.
+//!
+//! On the *cold path* (migration in progress) this middleware does:
+//!   1. the atomic load above;
+//!   2. one `RwLock::read` (uncontended — writers are the migration
+//!      handler, one per batch every ~100 blobs);
+//!   3. one small `serde_json::to_string` call on a 4-field struct
+//!      (a few dozen bytes);
+//!   4. one header insertion.
+//!
+//! Total per-request work in this branch: microseconds.
+
+use axum::extract::Request;
+use axum::extract::State;
+use axum::http::HeaderValue;
+use axum::middleware::Next;
+use axum::response::Response;
+use std::sync::Arc;
+use std::sync::atomic::Ordering;
+
+use crate::common::di::AppState;
+
+/// Name of the response header the frontend reads. Kept short — an
+/// admin browser session may keep this header around in every open
+/// tab's dev-tools network view during a migration; the value is
+/// small JSON but the name should not add bloat.
+pub const SERVER_STATUS_HEADER: &str = "x-server-status";
+
+/// Compact JSON shape written into the header. Fields are documented
+/// in `common::migration_progress::MigrationProgress`.
+///
+/// Kept internal so the wire format can evolve. Frontend treats the
+/// header as opaque JSON and pattern-matches on the fields it
+/// currently understands.
+#[derive(serde::Serialize)]
+struct HeaderPayload {
+    readonly: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    migration: Option<MigrationHeader>,
+}
+
+#[derive(serde::Serialize)]
+struct MigrationHeader {
+    // `target` is owned here — the RwLock guard is released before
+    // serialisation, so a borrowed slice wouldn't survive. Names
+    // are small (`[a-z0-9_-]{1,32}`) so the copy is trivial.
+    target: String,
+    migrated: u64,
+    total: u64,
+    percent: u8,
+}
+
+pub async fn server_status_middleware(
+    State(state): State<Arc<AppState>>,
+    request: Request,
+    next: Next,
+) -> Response {
+    // Hot-path fast return. When no migration is running the flag is
+    // false and there's nothing to emit — a bare atomic load and out.
+    let readonly = state.migration_readonly.load(Ordering::Relaxed);
+    let mut response = next.run(request).await;
+    if !readonly {
+        return response;
+    }
+
+    // Cold path — build the payload from the shared progress
+    // snapshot. If the snapshot is absent (readonly is true but the
+    // handler hasn't seeded progress yet, or a restart-during-
+    // migration scenario) we still emit `readonly: true` so the
+    // banner shows — the frontend renders a "maintenance in progress"
+    // message even when specific numbers aren't available.
+    let payload = {
+        let guard = state
+            .migration_progress
+            .read()
+            .unwrap_or_else(std::sync::PoisonError::into_inner);
+        HeaderPayload {
+            readonly: true,
+            migration: guard.as_ref().map(|p| MigrationHeader {
+                target: p.target_name.clone(),
+                migrated: p.migrated_blobs,
+                total: p.total_blobs,
+                percent: p.percent,
+            }),
+        }
+    };
+
+    // `serde_json::to_string` on this 4-field struct is a few
+    // dozen-byte allocation — negligible against the response body.
+    // A serialize failure here would be a programming bug (all
+    // fields are trivially serializable), so we degrade to a
+    // minimal `readonly: true` string rather than skipping the
+    // header entirely.
+    let value =
+        serde_json::to_string(&payload).unwrap_or_else(|_| r#"{"readonly":true}"#.to_string());
+    if let Ok(header_value) = HeaderValue::from_str(&value) {
+        response
+            .headers_mut()
+            .insert(SERVER_STATUS_HEADER, header_value);
+    }
+    response
+}