feat(flutter): per-item rendering infrastructure (Slice A)

Plumbing for the per-item rendering pass — no UI changes yet, just
the layers the home/playlist/liked migrations will sit on.

* CachedHomeIndex drift table (schema 5→6) — section/position →
  entity-id rows, populated by the upcoming /api/home/index endpoint.
* HydrationQueue (concurrency=4, in-flight dedup) — bounded request
  pump that takes (entityType, entityId) and persists the result to
  the right cached_<entity> table. Albums + artists wired today;
  tracks deferred until /api/tracks/:id exists.
* Per-entity tile providers (albumTileProvider, artistTileProvider,
  trackTileProvider as StreamProvider.family) — watch drift, enqueue
  hydration on miss, yield AsyncValue<EntityRef?>.
* Skeleton widgets (album/artist/track) matched to the real card
  dimensions with a 1.2s shimmer sweep using FabledSword tokens. No
  shimmer-package dep — single AnimationController per surface.

See docs/superpowers/specs/2026-05-13-per-item-rendering-design.md
for the full architecture rationale.

flutter_client/lib/cache/hydration_queue.dart

@@ -0,0 +1,137 @@
+// Per-item hydration coordinator for the home/playlist/liked surfaces.
+//
+// Tile widgets are reactive to their per-entity drift row. On first
+// build, if drift has no row for the requested id, the tile asks this
+// queue to hydrate it. The queue dispatches one /api/<type>/:id
+// request, writes the result to drift, and the drift watch() on the
+// tile re-emits with the populated row.
+//
+// Concurrency cap: HydrationQueue limits the number of in-flight
+// requests so a 50-tile home screen doesn't fire 50 parallel /api/
+// calls and stall the auth-token-bound dio pool. In-flight dedup
+// (keyed by `<type>:<id>`) ensures a single hydration when multiple
+// tiles share an entity (rare on home, common when several screens
+// reference the same album).
+//
+// Failures are swallowed — a single failed hydration leaves the tile
+// in skeleton state. A future retry-on-visit pass can layer on top
+// without changing the queue's contract.
+
+import 'dart:async';
+import 'dart:collection';
+
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../api/endpoints/library.dart';
+import '../cache/adapters.dart';
+import '../cache/audio_cache_manager.dart' show appDbProvider;
+import '../cache/db.dart';
+import '../library/library_providers.dart' show libraryApiProvider;
+
+/// Bounded-concurrency request queue for per-entity hydration. One
+/// instance per Riverpod scope (provider singleton below).
+class HydrationQueue {
+  HydrationQueue(this._ref);
+  final Ref _ref;
+
+  /// Concurrent slot count. 4 keeps the dio token pool from saturating
+  /// while still pulling tiles down faster than a serial loop. Bump
+  /// if profiling shows the queue idle while the network is healthy.
+  static const _maxConcurrent = 4;
+
+  final Queue<_HydrationRequest> _pending = Queue();
+  final Set<String> _inFlightKeys = {};
+  int _inFlight = 0;
+
+  /// Request hydration for (entityType, entityId). Idempotent — second
+  /// call for the same key while the first is queued or in flight is
+  /// dropped, so multiple tile rebuilds during a fast scroll don't
+  /// inflate the queue.
+  void enqueue({required String entityType, required String entityId}) {
+    if (entityId.isEmpty) return;
+    final key = '$entityType:$entityId';
+    if (_inFlightKeys.contains(key)) return;
+    if (_pending.any((r) => r.key == key)) return;
+    _pending.add(_HydrationRequest(
+        entityType: entityType, entityId: entityId, key: key));
+    _pump();
+  }
+
+  void _pump() {
+    while (_inFlight < _maxConcurrent && _pending.isNotEmpty) {
+      final req = _pending.removeFirst();
+      _inFlightKeys.add(req.key);
+      _inFlight++;
+      // Unawaited on purpose — _pump returns immediately so further
+      // enqueues can fill remaining slots while this one runs.
+      unawaited(_execute(req).whenComplete(() {
+        _inFlight--;
+        _inFlightKeys.remove(req.key);
+        _pump();
+      }));
+    }
+  }
+
+  Future<void> _execute(_HydrationRequest req) async {
+    try {
+      switch (req.entityType) {
+        case 'album':
+          await _hydrateAlbum(req.entityId);
+        case 'artist':
+          await _hydrateArtist(req.entityId);
+        case 'track':
+          // TODO(per-item slice B): GET /api/tracks/:id doesn't exist
+          // yet; tracks remain bulk-loaded until that endpoint lands.
+          // Tile providers for tracks read drift but never enqueue.
+          break;
+      }
+    } catch (_) {
+      // Swallow — tile stays in skeleton. A retry-on-visit pass can
+      // layer on top later without changing this contract.
+    }
+  }
+
+  Future<void> _hydrateAlbum(String id) async {
+    final api = await _ref.read(libraryApiProvider.future);
+    final result = await api.getAlbum(id);
+    final db = _ref.read(appDbProvider);
+    // Album + tracks come bundled in the same response; persist both
+    // so opening the album detail later is also a drift hit.
+    await db.transaction(() async {
+      await db
+          .into(db.cachedAlbums)
+          .insertOnConflictUpdate(result.album.toDrift());
+      if (result.tracks.isNotEmpty) {
+        await db.batch((b) {
+          b.insertAllOnConflictUpdate(
+              db.cachedTracks, result.tracks.map((t) => t.toDrift()).toList());
+        });
+      }
+    });
+  }
+
+  Future<void> _hydrateArtist(String id) async {
+    final api = await _ref.read(libraryApiProvider.future);
+    final fresh = await api.getArtist(id);
+    final db = _ref.read(appDbProvider);
+    await db.into(db.cachedArtists).insertOnConflictUpdate(fresh.toDrift());
+  }
+}
+
+class _HydrationRequest {
+  _HydrationRequest({
+    required this.entityType,
+    required this.entityId,
+    required this.key,
+  });
+  final String entityType;
+  final String entityId;
+  final String key;
+}
+
+/// Singleton queue scoped to the Riverpod container. Kept as a plain
+/// Provider (not StateProvider) since the queue's internal state is
+/// pump-driven, not reactively observed.
+final hydrationQueueProvider = Provider<HydrationQueue>((ref) {
+  return HydrationQueue(ref);
+});