perf: round 10 — auth alloc purge, parent-herd batching, query-shape pack, NC 304s

Benchmark-gated (benches/ROUND10.md; every change carries a BEFORE/AFTER
harness with equivalence/safety gates — two designs were rejected or
rewritten by their own benches before adoption):

- Auth hot path: TokenClaims/CurrentUser display fields to Arc<str>, role
  to inline SmolStr end-to-end (Bearer, cookie, Basic-auth cache) — 4→1
  allocs per authenticated request, 3→0 per warm DAV request; JWT
  Encoding/Decoding/Validation built once.
- Cold shared-album herd: leader-inline parent batching in PgAclEngine
  (+ cascade try_get_with single-flight) — 100→2 parent queries per
  100-thumb cold herd, herd wall 1.9x, sequential + warm paths unchanged,
  all ROUND8/9 safety gates plus new herd-equivalence gates.
- Query-shape pack: share download double-fetch 2→1 (2.18x), contact-group
  COUNT(*) 14.9x, save_faces UNNEST 3.9x, playlist reorder UNNEST 63.7x
  (now atomic), search files∥folders join! 1.45x, move drive-lookup join!
  2.14x, trash partial (drive_id, trashed_at) indexes, CalDAV event-gate
  narrow read, favorites/recents binary-decode port, dead count_files
  removed.
- NC surface: preview + avatar honour If-None-Match (e2e: 5 KB and 197 KB
  → 0 bytes per revalidation), avatar WebP→PNG transcode memoised,
  PROPFIND/trashbin integer+date emits on stack formatters, folder-header
  enrichment join!, chunk-PUT retry stat folded into create_new open.
- common::fmt integer rendering rewritten on the std 2-digit LUT after the
  round's own bench caught the div-loop losing to to_string (16.1 ns vs
  22.5; speeds every prior-round call site).
- Micro-pack: WebDAV scope probe borrow-only, ShareService base_url
  snapshot, cookie_secure OnceLock, Arc'd AES-GCM cipher, stack request-id,
  tantivy analyzer clone dropped.
- SPA: search stale-guard + AbortController (10→1 completed round-trips,
  stale-clobber gone), getFolder in-flight dedup, gridColumns matchMedia
  hoist (10k→0 style reads).

Backend: cargo fmt + clippy -D warnings clean, 524 tests green.
Frontend: npm run check clean, 301 vitest green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018DdM7V7M3QPW7HEHg3gLov

src/application/dtos/user_dto.rs

@@ -1,6 +1,8 @@
 use crate::domain::entities::user::User;
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
+use smol_str::SmolStr;
+use std::sync::Arc;
 use utoipa::ToSchema;
 use uuid::Uuid;
 
@@ -247,12 +249,21 @@ pub struct UpgradeToInternalDto {
 }
 
 /// Authenticated current user data (for use in application services)
+///
+/// Built once per authenticated request in the auth middlewares.
+/// `username`/`email` are `Arc<str>` (refcount-bump clones from the cached
+/// `TokenClaims` / Basic-auth cache — JSON shape unchanged) and `role` is an
+/// inline `SmolStr` ("admin"/"user" fit the 23-byte inline buffer, so the
+/// per-request live-role render allocates nothing).
 #[derive(Clone, Debug, Serialize, Deserialize, ToSchema)]
 pub struct CurrentUser {
     pub id: Uuid,
-    pub username: String,
-    pub email: String,
-    pub role: String,
+    #[schema(value_type = String)]
+    pub username: Arc<str>,
+    #[schema(value_type = String)]
+    pub email: Arc<str>,
+    #[schema(value_type = String)]
+    pub role: SmolStr,
 }
 
 // ============================================================================