feat(cache): outbound mutation queue for offline-resilient REST

User intent (likes, hides, playlist adds, Lidarr requests, cancels)
now persists across network loss. Controllers write their optimistic
local state to drift first, then try the REST call; on failure
the call is enqueued in cached_mutations rather than rolled back.
MutationReplayer drains the queue on connectivity transitions and
a 1-minute periodic tick.

**Infrastructure (schema 8):**
* CachedMutations table — id / kind / payload (JSON) / createdAt
  / lastAttemptAt / attempts. Drop-after-5-attempts semantics: a
  permanently-failing mutation eventually drops, and next sync
  reconciles drift to the server's authoritative state.
* MutationQueue.enqueue / pendingCount
* MutationReplayer.start + .drain — start fires from app.dart's
  postFrameCallback alongside SyncController / Prefetcher / etc.
* Kind registry: like.add / like.remove / quarantine.flag /
  quarantine.unflag / playlist.append / request.create /
  request.cancel — each with a Ref+payload handler that re-fires
  the corresponding REST call.

**Wired surfaces:**
* LikesController.toggle — optimistic drift like/unlike stays
  across REST failure; queues the call. Drops the old rollback.
* MyQuarantineController.flag / .unflag — same pattern. Hide/unhide
  visibly persists offline; replays when back online.
* addToPlaylistActionProvider — now does an optimistic
  cached_playlist_tracks write (position = max + 1) so the
  playlist detail screen shows the new track instantly. Queues
  appendTracks on REST failure.
* DiscoverScreen._request — queues request.create on DioException.
  No drift state for the request itself (myRequestsProvider is
  still REST-only) so the row won't show on /requests until replay
  succeeds — acceptable for v1.
* MyRequestsController.cancel — optimistic in-memory remove no
  longer restores on failure; queues request.cancel instead.

**Test update:**
quarantine_provider_test "flag rolls back on server failure"
renamed and rewritten to assert the new offline behavior:
optimistic drift row persists, mutation is enqueued for replay.

**Out of scope (v2):**
* Playlist create / rename / delete (no Flutter UI exposes these yet)
* Lidarr request optimistic local row (would need a cached_requests
  drift table)
* UI "syncing N pending changes" indicator (operator preference:
  silent unless we find a concrete need)

flutter_client/lib/likes/likes_provider.dart

@@ -7,6 +7,7 @@ import '../cache/audio_cache_manager.dart' show appDbProvider;
 import '../cache/cache_first.dart';
 import '../cache/connectivity_provider.dart';
 import '../cache/db.dart';
+import '../cache/mutation_queue.dart';
 import '../library/library_providers.dart';
 
 final likesApiProvider = FutureProvider<LikesApi>((ref) async {
@@ -147,26 +148,18 @@ class LikesController {
       } else {
         await api.like(kind, id);
       }
-    } catch (e, st) {
-      // Rollback drift
-      if (wasLiked) {
-        await db.into(db.cachedLikes).insert(
-              CachedLikesCompanion.insert(
-                userId: user.id,
-                entityType: entityType,
-                entityId: id,
-              ),
-              mode: drift.InsertMode.insertOrIgnore,
-            );
-      } else {
-        await (db.delete(db.cachedLikes)
-              ..where((t) =>
-                  t.userId.equals(user.id) &
-                  t.entityType.equals(entityType) &
-                  t.entityId.equals(id)))
-            .go();
-      }
-      Error.throwWithStackTrace(e, st);
+    } catch (_) {
+      // REST failed (network or HTTP). Don't roll back drift — the
+      // user's intent is to like/unlike, and we want that visible to
+      // them even when offline. Queue the call for replay; the
+      // MutationReplayer will retry on next connectivity transition.
+      // If retries exhaust (5 attempts), the row drops and the next
+      // SyncController.sync brings drift back in line with the
+      // server's authoritative state.
+      await _ref.read(mutationQueueProvider).enqueue(
+        wasLiked ? MutationKinds.likeRemove : MutationKinds.likeAdd,
+        {'kind': entityType, 'id': id},
+      );
     }
   }