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/cache/db.dart

@@ -178,6 +178,30 @@ class CachedHomeIndex extends Table {
   Set<Column> get primaryKey => {section, position};
 }
 
+/// Outbound mutation queue for offline-resilient REST calls. Likes,
+/// quarantine flag/unflag, playlist appendTracks, Lidarr request
+/// create/cancel all enqueue here on REST failure so the user's
+/// intent persists across network loss. MutationReplayer drains on
+/// connectivity transitions and a 1-minute periodic tick; entries
+/// with attempts >= 5 are skipped (the server treats them as
+/// permanent failures and library_changes sync will correct any
+/// resulting drift drift on next sync). Schema 8+.
+class CachedMutations extends Table {
+  IntColumn get id => integer().autoIncrement()();
+  /// Stable kind string (see mutation_queue.dart constants). The
+  /// replayer looks up the handler in a kind→Function map; unknown
+  /// kinds are dropped to avoid getting stuck.
+  TextColumn get kind => text()();
+  /// JSON-encoded payload — args needed to replay the REST call.
+  /// Shape depends on kind; see mutation_queue.dart.
+  TextColumn get payload => text()();
+  DateTimeColumn get createdAt => dateTime().withDefault(currentDateAndTime)();
+  /// When the last drain attempt fired against this row. Null until
+  /// the first attempt. Useful for diagnostic queries.
+  DateTimeColumn get lastAttemptAt => dateTime().nullable()();
+  IntColumn get attempts => integer().withDefault(const Constant(0))();
+}
+
 /// Caller-scoped quarantine ("hidden") rows. Mirrors the wire shape of
 /// /api/quarantine/mine — fully denormalized (track + album + artist
 /// fields inline) because the Hidden tab renders straight from this row
@@ -217,12 +241,13 @@ enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
   CachedQuarantineMine,
   CachedHomeIndex,
   CachedSystemPlaylistsStatus,
+  CachedMutations,
 ])
 class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 7;
+  int get schemaVersion => 8;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -270,6 +295,11 @@ class AppDb extends _$AppDb {
             // populates.
             await m.createTable(cachedSystemPlaylistsStatus);
           }
+          if (from < 8) {
+            // Schema 8: cached_mutations outbound queue. Empty on
+            // upgrade; populated by controllers when REST calls fail.
+            await m.createTable(cachedMutations);
+          }
         },
       );
 }