feat(api): cursor listing contract — PageCursor trait + resource field

- Add src/application/dtos/cursor.rs with three shared types:
  · PageCursor trait  — default base64url+JSON encode/decode; one bare
    impl line per cursor struct
  · CursorQuery struct — standard limit/cursor/sort_by query params with
    limit_clamped() and decode_cursor<C>() helpers; compose via flatten
  · CursorListResponse<T> — standard {items, next_cursor?} envelope with
    from_oversized() and with_cursor() builders

- Migrate GrantCursor to impl PageCursor (remove duplicate encode/decode)

- Update GET /api/grants/incoming/resources:
  · SharedWithMeQuery now embeds CursorQuery via #[serde(flatten)]
  · Replace file/folder nullable pair with ResourceContentDto (untagged
    enum) under a single always-present 'resource' field
  · SharedWithMeDto is now a type alias for CursorListResponse<SharedWithMeItemDto>
  · Handler uses q.paging.limit_clamped() and decode_cursor<GrantCursor>()

- Add docs/architecture/resource-listing.md — authoritative contract for
  all listing endpoints (cursor design, SQL keyset WHERE, sort_by naming,
  Rust + JS skeletons, compliance table, migration guide)

- Register doc in VitePress sidebar and architecture index

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/application/dtos/cursor.rs

@@ -0,0 +1,161 @@
+//! Standard types for cursor-based, sortable listing endpoints.
+//!
+//! All `GET` endpoints that return a collection **must** use these types so
+//! that every listing is consistent for API consumers.
+//!
+//! # Quick start
+//!
+//! ```rust,ignore
+//! // 1. Define a cursor for your endpoint
+//! #[derive(Serialize, Deserialize)]
+//! pub struct MyCursor { pub created_at: DateTime<Utc>, pub id: Uuid }
+//! impl PageCursor for MyCursor {}          // encode/decode for free
+//!
+//! // 2. Compose the standard query params
+//! #[derive(Deserialize, IntoParams)]
+//! pub struct MyQuery {
+//!     #[serde(flatten)]
+//!     pub paging: CursorQuery,
+//!     pub my_filter: Option<String>,      // endpoint-specific extras
+//! }
+//!
+//! // 3. Return the standard envelope
+//! async fn list_things(Query(q): Query<MyQuery>, …) -> Json<CursorListResponse<ThingDto>> {
+//!     let limit  = q.paging.limit_clamped();
+//!     let cursor = q.paging.decode_cursor::<MyCursor>();
+//!     // fetch limit+1 rows …
+//!     Json(CursorListResponse::from_oversized(rows, limit, |r| MyCursor { … }))
+//! }
+//! ```
+
+use base64::{Engine as _, engine::general_purpose::URL_SAFE_NO_PAD};
+use serde::{Deserialize, Serialize};
+use utoipa::{IntoParams, ToSchema};
+// ToSchema is used on CursorQuery so it can be flattened into IntoParams structs
+
+// ════════════════════════════════════════════════════════════════════════════
+// PageCursor trait
+// ════════════════════════════════════════════════════════════════════════════
+
+/// Marker trait for opaque keyset-pagination cursors.
+///
+/// The default `encode` / `decode` implementations use
+/// URL-safe base64url (no padding) over a JSON serialisation of `Self`.
+/// Any struct that derives `Serialize + Deserialize` can implement this
+/// with a bare `impl PageCursor for MyCursor {}`.
+///
+/// The encoding is intentionally opaque to API callers. Treat an
+/// undecodable cursor as "start from the top" — never return an error.
+pub trait PageCursor: Sized + Serialize + for<'de> Deserialize<'de> {
+    /// Encode `self` as a URL-safe, no-padding base64url string.
+    fn encode(&self) -> String {
+        URL_SAFE_NO_PAD.encode(serde_json::to_vec(self).unwrap_or_default())
+    }
+
+    /// Decode from a base64url string.  Returns `None` on any parse failure.
+    fn decode(s: &str) -> Option<Self> {
+        let bytes = URL_SAFE_NO_PAD.decode(s).ok()?;
+        serde_json::from_slice(&bytes).ok()
+    }
+}
+
+// ════════════════════════════════════════════════════════════════════════════
+// CursorQuery — standard query params
+// ════════════════════════════════════════════════════════════════════════════
+
+/// Standard query parameters for cursor-based listing endpoints.
+///
+/// Use `CursorQuery` directly as the `Query<CursorQuery>` extractor when an
+/// endpoint has no extra filter params.  When extra params are needed, declare
+/// them in an endpoint-specific struct and **repeat** the three fields — Axum's
+/// query extractor uses `serde_urlencoded` which does not support
+/// `#[serde(flatten)]`.  Use `CursorQuery::default_limit()` for the default
+/// and the helpers `limit_clamped()` / `decode_cursor()` by either calling
+/// them on `CursorQuery` directly or re-implementing them inline:
+///
+/// ```rust,ignore
+/// #[derive(Deserialize, IntoParams)]
+/// pub struct MyQuery {
+///     #[serde(default = "CursorQuery::default_limit")]
+///     pub limit: u32,
+///     pub cursor: Option<String>,
+///     pub sort_by: Option<String>,
+///     pub status: Option<String>,    // endpoint-specific
+/// }
+/// ```
+#[derive(Debug, Deserialize, IntoParams, ToSchema)]
+pub struct CursorQuery {
+    /// Maximum items per page (1–200, default 50).
+    #[serde(default = "CursorQuery::default_limit")]
+    pub limit: u32,
+    /// Opaque cursor from a previous response. Absent on the first page.
+    pub cursor: Option<String>,
+    /// Sort dimension.  Valid values are endpoint-defined (e.g. `"granted_at"`,
+    /// `"name"`, `"granted_by"`).  Unknown values should return HTTP 400.
+    pub sort_by: Option<String>,
+}
+
+impl CursorQuery {
+    /// Default value for the `limit` field — exposed `pub` so endpoint-specific
+    /// query structs can reference it in `#[serde(default = "CursorQuery::default_limit")]`.
+    pub fn default_limit() -> u32 {
+        50
+    }
+
+    /// Returns `limit` clamped to `[1, 200]`.
+    pub fn limit_clamped(&self) -> usize {
+        self.limit.clamp(1, 200) as usize
+    }
+
+    /// Decode the optional cursor string into type `C`.
+    /// Returns `None` when no cursor is present or when decoding fails
+    /// (invalid cursor → start from the top).
+    pub fn decode_cursor<C: PageCursor>(&self) -> Option<C> {
+        self.cursor.as_deref().and_then(C::decode)
+    }
+}
+
+// ════════════════════════════════════════════════════════════════════════════
+// CursorListResponse — standard response envelope
+// ════════════════════════════════════════════════════════════════════════════
+
+/// Standard response envelope for cursor-paginated listing endpoints.
+///
+/// `next_cursor` is omitted from the JSON when `None` (i.e. last page).
+/// Callers must treat a missing `next_cursor` as end-of-results — never
+/// include a `total` count (that would require an expensive `COUNT(*)`).
+#[derive(Debug, Serialize, ToSchema)]
+pub struct CursorListResponse<T: Serialize> {
+    pub items: Vec<T>,
+    /// Opaque cursor for the next page.  Absent when this is the last page.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub next_cursor: Option<String>,
+}
+
+impl<T: Serialize> CursorListResponse<T> {
+    /// Build a response from an over-fetched slice (fetch `limit + 1` rows).
+    ///
+    /// If `items.len() > limit` a next page exists: `items` is truncated to
+    /// `limit` and `cursor_fn` is called on the **last kept item** to produce
+    /// the next cursor.  Otherwise `next_cursor` is `None`.
+    pub fn from_oversized<C: PageCursor>(
+        mut items: Vec<T>,
+        limit: usize,
+        cursor_fn: impl FnOnce(&T) -> C,
+    ) -> Self {
+        let next_cursor = if items.len() > limit {
+            let c = cursor_fn(&items[limit - 1]);
+            items.truncate(limit);
+            Some(c.encode())
+        } else {
+            None
+        };
+        Self { items, next_cursor }
+    }
+
+    /// Build a response when the next cursor is already known (e.g. returned
+    /// by a service layer that handles the `limit+1` logic internally).
+    pub fn with_cursor(items: Vec<T>, next_cursor: Option<String>) -> Self {
+        Self { items, next_cursor }
+    }
+}

src/application/dtos/grant_dto.rs

@@ -8,6 +8,7 @@ use serde::{Deserialize, Serialize};
 use utoipa::{IntoParams, ToSchema};
 use uuid::Uuid;
 
+use crate::application::dtos::cursor::{CursorListResponse, CursorQuery, PageCursor};
 use crate::application::dtos::file_dto::FileDto;
 use crate::application::dtos::folder_dto::FolderDto;
 use crate::domain::services::authorization::{Grant, Permission, Resource, Subject};
@@ -232,26 +233,58 @@ impl From<Grant> for GrantDto {
 // ════════════════════════════════════════════════════════════════════════════
 
 /// Query parameters for `GET /api/grants/incoming/resources`.
+///
+/// `limit`, `cursor`, and `sort_by` follow the standard [`CursorQuery`]
+/// contract.  They are declared directly here rather than via
+/// `#[serde(flatten)]` because `serde_urlencoded` (Axum's query extractor)
+/// does not support flattening.
 #[derive(Debug, Deserialize, IntoParams)]
 pub struct SharedWithMeQuery {
     /// Maximum number of items to return (1–200, default 50).
-    #[serde(default = "shared_with_me_default_limit")]
+    #[serde(default = "CursorQuery::default_limit")]
     pub limit: u32,
+    /// Opaque cursor from a previous response. Omit to start from the
+    /// most-recently-granted item.
+    pub cursor: Option<String>,
+    /// Sort dimension. Supported values: `"granted_at"` (default),
+    /// `"granted_by"` (for swimlane grouping).
+    pub sort_by: Option<String>,
     /// Comma-separated resource types to include, e.g. `file,folder`.
     /// Omit to return all known types.
     pub resource_types: Option<String>,
-    /// Opaque cursor returned by a previous call. Omit to start from the
-    /// most-recently-granted item.
-    pub cursor: Option<String>,
 }
 
-fn shared_with_me_default_limit() -> u32 {
-    50
+impl SharedWithMeQuery {
+    /// Returns `limit` clamped to `[1, 200]`.
+    pub fn limit_clamped(&self) -> usize {
+        self.limit.clamp(1, 200) as usize
+    }
+
+    /// Decode the optional cursor string.  Invalid cursor → start from top.
+    pub fn decode_cursor<C: PageCursor>(&self) -> Option<C> {
+        self.cursor.as_deref().and_then(C::decode)
+    }
 }
 
-/// One item in the shared-with-me list. Exactly one of `file` / `folder` is
-/// populated, indicated by `resource_type`. Additional optional fields for
-/// future resource types (playlist, addressbook, …) will be added here.
+/// The resource payload for one item in the shared-with-me list.
+///
+/// The variant is discriminated by `resource_type` on the parent
+/// [`SharedWithMeItemDto`].  Serialised as the inner object (no wrapper key)
+/// via `#[serde(untagged)]`, so consumers see the file/folder fields directly
+/// under the `resource` key.
+#[derive(Debug, Serialize, ToSchema)]
+#[serde(untagged)]
+pub enum ResourceContentDto {
+    File(FileDto),
+    Folder(FolderDto),
+}
+
+/// One item in the shared-with-me list.
+///
+/// `resource_type` indicates whether `resource` contains a file or a folder.
+/// Using a single `resource` field (instead of nullable `file`/`folder` pairs)
+/// makes adding new resource types backward-compatible — only `resource_type`
+/// gains a new variant; the wrapper shape stays the same.
 #[derive(Debug, Serialize, ToSchema)]
 pub struct SharedWithMeItemDto {
     pub resource_type: ResourceTypeDto,
@@ -261,17 +294,9 @@ pub struct SharedWithMeItemDto {
     pub granted_at: chrono::DateTime<chrono::Utc>,
     /// UUID of the user who created the (earliest) grant.
     pub granted_by: Uuid,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub file: Option<FileDto>,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub folder: Option<FolderDto>,
+    /// Full resource details. Shape is determined by `resource_type`.
+    pub resource: ResourceContentDto,
 }
 
 /// Response for `GET /api/grants/incoming/resources`.
-#[derive(Debug, Serialize, ToSchema)]
-pub struct SharedWithMeDto {
-    pub items: Vec<SharedWithMeItemDto>,
-    /// Opaque cursor for the next page. Absent when the last page is reached.
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub next_cursor: Option<String>,
-}
+pub type SharedWithMeDto = CursorListResponse<SharedWithMeItemDto>;

src/application/dtos/mod.rs

@@ -1,4 +1,6 @@
 pub mod address_book_dto;
+pub mod cursor;
+pub use cursor::{CursorListResponse, CursorQuery, PageCursor};
 pub mod app_password_dto;
 pub mod calendar_dto;
 pub mod contact_dto;