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/app.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'cache/cache_filler.dart';
 import 'cache/metadata_prefetcher.dart';
+import 'cache/mutation_queue.dart';
 import 'cache/prefetcher.dart';
 import 'cache/sync_controller.dart';
 import 'shared/live_events_dispatcher.dart';
@@ -48,6 +49,12 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
       // thereafter. Throttled 200ms between requests so it never
       // competes with user activity.
       ref.read(cacheFillerProvider);
+      // Mutation replayer: drains the cached_mutations queue when
+      // connectivity comes back. Controllers (LikesController,
+      // MyQuarantineController, the add-to-playlist + request flows)
+      // enqueue on REST failure so user intent persists across
+      // network loss instead of getting rolled back.
+      ref.read(mutationReplayerProvider);
     });
   }
 

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);
+          }
         },
       );
 }

flutter_client/lib/cache/mutation_queue.dart

@@ -0,0 +1,298 @@
+// Outbound mutation queue for offline-resilient REST calls.
+//
+// Controllers (LikesController, MyQuarantineController, the add-to-
+// playlist sheet, the Discover request flow, the Requests cancel
+// action) write their optimistic local state to drift first and try
+// the corresponding REST call. On failure they enqueue here. The
+// replayer drains the queue when connectivity comes back; the user's
+// intent persists across network loss without rolling back their
+// visible action.
+//
+// Why this is separate from SyncController: sync ingests AUTHORITATIVE
+// SERVER state into drift. The mutation queue carries USER INTENT
+// outbound to the server. They flow in opposite directions and
+// shouldn't be conflated — sync wins on conflict (server is the
+// source of truth), and a mutation that fails forever (5 attempts)
+// is dropped on the trust that next sync will reconcile drift to
+// match the server's actual state.
+//
+// Drop semantics: 5 attempts then drop. The drop is intentional —
+// holding onto a forever-failing mutation just delays sync's
+// reconciliation. The user's optimistic drift state will diverge
+// from server, and the next library_changes delta corrects it.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:dio/dio.dart' show DioException, DioExceptionType;
+import 'package:drift/drift.dart';
+import 'package:flutter/foundation.dart' show debugPrint;
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../api/endpoints/discover.dart';
+import '../api/endpoints/likes.dart';
+import '../library/library_providers.dart' show dioProvider;
+import '../likes/likes_provider.dart' show likesApiProvider;
+import '../models/lidarr.dart' show LidarrRequestKind;
+import '../playlists/playlists_provider.dart' show playlistsApiProvider;
+import '../quarantine/quarantine_provider.dart' show quarantineApiProvider;
+import '../requests/requests_provider.dart' show requestsApiProvider;
+import 'audio_cache_manager.dart' show appDbProvider;
+import 'connectivity_provider.dart';
+import 'db.dart';
+
+/// Stable kind constants. Each controller emits one of these via
+/// MutationQueue.enqueue; the replayer dispatches by kind.
+class MutationKinds {
+  MutationKinds._();
+  static const likeAdd = 'like.add';
+  static const likeRemove = 'like.remove';
+  static const quarantineFlag = 'quarantine.flag';
+  static const quarantineUnflag = 'quarantine.unflag';
+  static const playlistAppend = 'playlist.append';
+  static const requestCreate = 'request.create';
+  static const requestCancel = 'request.cancel';
+}
+
+class MutationQueue {
+  MutationQueue(this._ref);
+  final Ref _ref;
+
+  /// Persist a mutation for replay. Caller has already done the
+  /// optimistic local mutation (drift write, in-memory state update)
+  /// — this only records the REST call that needs to fire eventually.
+  Future<void> enqueue(String kind, Map<String, dynamic> payload) async {
+    final db = _ref.read(appDbProvider);
+    await db.into(db.cachedMutations).insert(
+          CachedMutationsCompanion.insert(
+            kind: kind,
+            payload: jsonEncode(payload),
+          ),
+        );
+    // Nudge the replayer in case we're online right now.
+    unawaited(_ref.read(mutationReplayerProvider).drain());
+  }
+
+  /// Count of pending mutations (attempts < 5). Diagnostic / future
+  /// UI surface for a "syncing N pending changes" indicator.
+  Future<int> pendingCount() async {
+    final db = _ref.read(appDbProvider);
+    final rows = await db.customSelect(
+      'SELECT COUNT(*) AS c FROM cached_mutations WHERE attempts < 5',
+    ).get();
+    return rows.first.read<int>('c');
+  }
+}
+
+final mutationQueueProvider =
+    Provider<MutationQueue>((ref) => MutationQueue(ref));
+
+class MutationReplayer {
+  MutationReplayer(this._ref);
+  final Ref _ref;
+
+  Timer? _timer;
+  bool _running = false;
+  bool _disposed = false;
+
+  /// Periodic poll cadence. The connectivity listener catches most
+  /// transitions; this is the belt-and-suspenders for cases where
+  /// connectivity_plus misses a state change (e.g. flaky Wi-Fi).
+  static const _interval = Duration(minutes: 1);
+
+  /// Max replay attempts before we drop a mutation. 5 is enough to
+  /// span a few reconnection cycles without holding onto a forever-
+  /// broken request. See file header for the drop rationale.
+  static const _maxAttempts = 5;
+
+  void start() {
+    // Drain shortly after launch in case there are queued mutations
+    // from a prior session that died offline. 3s lets the auth +
+    // server-url + dio providers finish their async warmup.
+    Timer(const Duration(seconds: 3), drain);
+
+    // Drain on every connectivity emission that resolves to online.
+    // ref.listen fires on changes after subscription; we don't get
+    // the initial value, but the Timer above + the periodic poll
+    // cover that.
+    _ref.listen(connectivityProvider, (prev, next) {
+      final isOnline = next.value ?? false;
+      if (isOnline) unawaited(drain());
+    });
+
+    _timer = Timer.periodic(_interval, (_) => drain());
+  }
+
+  /// Walk the queue oldest-first, replaying each mutation. Stops on
+  /// any transient network error (we'll retry on the next tick); a
+  /// permanent error (404 etc.) increments the attempt counter and
+  /// moves on to the next mutation. This is public so MutationQueue.
+  /// enqueue can trigger an immediate attempt after writing.
+  Future<void> drain() async {
+    if (_disposed || _running) return;
+    final online = await _ref
+        .read(connectivityProvider.future)
+        .timeout(const Duration(seconds: 3), onTimeout: () => true);
+    if (!online) return;
+    _running = true;
+    try {
+      while (!_disposed) {
+        final db = _ref.read(appDbProvider);
+        final next = await (db.select(db.cachedMutations)
+              ..where((t) => t.attempts.isSmallerThanValue(_maxAttempts))
+              ..orderBy([(t) => OrderingTerm.asc(t.createdAt)])
+              ..limit(1))
+            .getSingleOrNull();
+        if (next == null) break;
+
+        final handler = _handlers[next.kind];
+        if (handler == null) {
+          debugPrint('mutation_replayer: unknown kind ${next.kind}, dropping');
+          await (db.delete(db.cachedMutations)
+                ..where((t) => t.id.equals(next.id)))
+              .go();
+          continue;
+        }
+
+        Map<String, dynamic> payload;
+        try {
+          payload = jsonDecode(next.payload) as Map<String, dynamic>;
+        } catch (e) {
+          debugPrint('mutation_replayer: bad payload for ${next.kind}: $e');
+          await (db.delete(db.cachedMutations)
+                ..where((t) => t.id.equals(next.id)))
+              .go();
+          continue;
+        }
+
+        try {
+          await handler(_ref, payload);
+          await (db.delete(db.cachedMutations)
+                ..where((t) => t.id.equals(next.id)))
+              .go();
+        } on DioException catch (e) {
+          await (db.update(db.cachedMutations)
+                ..where((t) => t.id.equals(next.id)))
+              .write(CachedMutationsCompanion(
+            attempts: Value(next.attempts + 1),
+            lastAttemptAt: Value(DateTime.now()),
+          ));
+          if (_isTransient(e)) {
+            // Network is flaky again — bail and try the whole queue
+            // later. The current mutation will be retried next tick.
+            break;
+          }
+          // Permanent failure (4xx/5xx with a real response). Leave
+          // the row with bumped attempts and move on to the next so
+          // one bad mutation doesn't block the others.
+        } catch (e, st) {
+          debugPrint('mutation_replayer: unexpected error: $e\n$st');
+          await (db.update(db.cachedMutations)
+                ..where((t) => t.id.equals(next.id)))
+              .write(CachedMutationsCompanion(
+            attempts: Value(next.attempts + 1),
+            lastAttemptAt: Value(DateTime.now()),
+          ));
+        }
+      }
+    } finally {
+      _running = false;
+    }
+  }
+
+  bool _isTransient(DioException e) {
+    switch (e.type) {
+      case DioExceptionType.connectionError:
+      case DioExceptionType.connectionTimeout:
+      case DioExceptionType.sendTimeout:
+      case DioExceptionType.receiveTimeout:
+        return true;
+      default:
+        return false;
+    }
+  }
+
+  void dispose() {
+    _disposed = true;
+    _timer?.cancel();
+  }
+}
+
+final mutationReplayerProvider = Provider<MutationReplayer>((ref) {
+  final r = MutationReplayer(ref);
+  ref.onDispose(r.dispose);
+  r.start();
+  return r;
+});
+
+/// Type signature for kind-specific replay handlers.
+typedef _Handler = Future<void> Function(Ref, Map<String, dynamic>);
+
+/// Kind → handler dispatch. Each handler decodes its payload and
+/// re-fires the original REST call. Throws on failure (caught by the
+/// replayer's drain loop above).
+///
+/// Payload shapes (see also enqueue call sites for the writer side):
+/// * like.add / like.remove: {'kind': 'track'|'album'|'artist', 'id': uuid}
+/// * quarantine.flag: {'trackId': uuid, 'reason': str, 'notes': str}
+/// * quarantine.unflag: {'trackId': uuid}
+/// * playlist.append: {'playlistId': uuid, 'trackIds': [uuid, …]}
+/// * request.create: {full createRequest args; see DiscoverApi}
+/// * request.cancel: {'id': uuid}
+final Map<String, _Handler> _handlers = {
+  MutationKinds.likeAdd: (ref, p) async {
+    final api = await ref.read(likesApiProvider.future);
+    await api.like(_likeKindFromString(p['kind'] as String), p['id'] as String);
+  },
+  MutationKinds.likeRemove: (ref, p) async {
+    final api = await ref.read(likesApiProvider.future);
+    await api.unlike(
+        _likeKindFromString(p['kind'] as String), p['id'] as String);
+  },
+  MutationKinds.quarantineFlag: (ref, p) async {
+    final api = await ref.read(quarantineApiProvider.future);
+    await api.flag(
+      p['trackId'] as String,
+      p['reason'] as String,
+      notes: (p['notes'] as String?) ?? '',
+    );
+  },
+  MutationKinds.quarantineUnflag: (ref, p) async {
+    final api = await ref.read(quarantineApiProvider.future);
+    await api.unflag(p['trackId'] as String);
+  },
+  MutationKinds.playlistAppend: (ref, p) async {
+    final api = await ref.read(playlistsApiProvider.future);
+    final ids = (p['trackIds'] as List).cast<String>();
+    await api.appendTracks(p['playlistId'] as String, ids);
+  },
+  MutationKinds.requestCreate: (ref, p) async {
+    final dio = await ref.read(dioProvider.future);
+    final api = DiscoverApi(dio);
+    await api.createRequest(
+      kind: _lidarrKindFromString(p['kind'] as String),
+      artistMbid: p['artistMbid'] as String,
+      artistName: p['artistName'] as String,
+      albumMbid: p['albumMbid'] as String?,
+      albumTitle: p['albumTitle'] as String?,
+      trackMbid: p['trackMbid'] as String?,
+      trackTitle: p['trackTitle'] as String?,
+    );
+  },
+  MutationKinds.requestCancel: (ref, p) async {
+    final api = await ref.read(requestsApiProvider.future);
+    await api.cancel(p['id'] as String);
+  },
+};
+
+LikeKind _likeKindFromString(String s) => switch (s) {
+      'album' => LikeKind.album,
+      'artist' => LikeKind.artist,
+      _ => LikeKind.track,
+    };
+
+LidarrRequestKind _lidarrKindFromString(String s) => switch (s) {
+      'album' => LidarrRequestKind.album,
+      'track' => LidarrRequestKind.track,
+      _ => LidarrRequestKind.artist,
+    };

flutter_client/lib/discover/discover_screen.dart

@@ -6,6 +6,7 @@ import 'package:go_router/go_router.dart';
 
 import '../api/endpoints/discover.dart';
 import '../api/errors.dart';
+import '../cache/mutation_queue.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/lidarr.dart';
 import '../shared/widgets/main_app_bar_actions.dart';
@@ -48,14 +49,23 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
 
   Future<void> _request(LidarrSearchResult row) async {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final args = {
+      'kind': _kind.wire,
+      'artistMbid':
+          _kind == LidarrRequestKind.artist ? row.mbid : row.artistMbid,
+      'artistName':
+          _kind == LidarrRequestKind.artist ? row.name : row.secondaryText,
+      'albumMbid': _kind == LidarrRequestKind.album ? row.mbid : null,
+      'albumTitle': _kind == LidarrRequestKind.album ? row.name : null,
+    };
     try {
       final api = await ref.read(_discoverApiProvider.future);
       await api.createRequest(
         kind: _kind,
-        artistMbid: _kind == LidarrRequestKind.artist ? row.mbid : row.artistMbid,
-        artistName: _kind == LidarrRequestKind.artist ? row.name : row.secondaryText,
-        albumMbid: _kind == LidarrRequestKind.album ? row.mbid : null,
-        albumTitle: _kind == LidarrRequestKind.album ? row.name : null,
+        artistMbid: args['artistMbid'] as String,
+        artistName: args['artistName'] as String,
+        albumMbid: args['albumMbid'] as String?,
+        albumTitle: args['albumTitle'] as String?,
       );
       if (mounted) {
         ScaffoldMessenger.of(context).showSnackBar(SnackBar(
@@ -65,12 +75,18 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
         // Re-run search to refresh the `requested` flag on the row.
         _runSearch();
       }
-    } on DioException catch (e) {
+    } on DioException catch (_) {
+      // Queue for replay so the user's request persists across
+      // network loss. We don't have a drift table for in-flight
+      // requests yet, so the row won't show on the Requests screen
+      // until replay succeeds — accept that for v1.
+      await ref
+          .read(mutationQueueProvider)
+          .enqueue(MutationKinds.requestCreate, args);
       if (mounted) {
-        final code = ApiError.fromDio(e).code;
         ScaffoldMessenger.of(context).showSnackBar(SnackBar(
-          content: Text('Request failed: $code'),
-          backgroundColor: fs.error,
+          content: Text('Request queued: ${row.name}'),
+          backgroundColor: fs.iron,
         ));
       }
     }

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},
+      );
     }
   }
 

flutter_client/lib/quarantine/quarantine_provider.dart

@@ -8,6 +8,7 @@ import '../api/endpoints/quarantine.dart';
 import '../cache/audio_cache_manager.dart' show appDbProvider;
 import '../cache/connectivity_provider.dart';
 import '../cache/db.dart';
+import '../cache/mutation_queue.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/quarantine_mine.dart';
 import '../models/track.dart';
@@ -110,17 +111,18 @@ class MyQuarantineController extends AsyncNotifier<List<QuarantineMineRow>> {
     try {
       final api = await ref.read(quarantineApiProvider.future);
       await api.flag(track.id, reason, notes: notes);
-    } catch (e, st) {
-      // Rollback the optimistic insert; watch() re-emits the prior set.
-      await (db.delete(db.cachedQuarantineMine)
-            ..where((t) => t.trackId.equals(track.id)))
-          .go();
-      Error.throwWithStackTrace(e, st);
+    } catch (_) {
+      // REST failed — don't roll back; queue for replay so the user's
+      // "hide this track" intent persists offline.
+      await ref.read(mutationQueueProvider).enqueue(
+        MutationKinds.quarantineFlag,
+        {'trackId': track.id, 'reason': reason, 'notes': notes},
+      );
     }
   }
 
-  /// Optimistic unflag: removes the drift row, calls server, restores
-  /// on error.
+  /// Optimistic unflag: removes the drift row, calls server, queues
+  /// the call on failure so the unhide intent persists offline.
   Future<void> unflag(String trackId) async {
     final db = ref.read(appDbProvider);
     final existing = await (db.select(db.cachedQuarantineMine)
@@ -135,13 +137,11 @@ class MyQuarantineController extends AsyncNotifier<List<QuarantineMineRow>> {
     try {
       final api = await ref.read(quarantineApiProvider.future);
       await api.unflag(trackId);
-    } catch (e, st) {
-      // Restore the row we just deleted — toCompanion(true) preserves
-      // every field including the original fetchedAt timestamp.
-      await db
-          .into(db.cachedQuarantineMine)
-          .insertOnConflictUpdate(existing.toCompanion(true));
-      Error.throwWithStackTrace(e, st);
+    } catch (_) {
+      // Don't restore the drift row; queue the unflag for replay.
+      await ref
+          .read(mutationQueueProvider)
+          .enqueue(MutationKinds.quarantineUnflag, {'trackId': trackId});
     }
   }
 

flutter_client/lib/requests/requests_provider.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../api/endpoints/requests.dart';
+import '../cache/mutation_queue.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/admin_request.dart';
 
@@ -41,17 +42,20 @@ class MyRequestsController extends AsyncNotifier<List<AdminRequest>> {
     }
   }
 
-  /// Optimistic remove + REST cancel. Restores the row on failure so
-  /// the user can retry — silent failure would lie about success.
+  /// Optimistic remove + REST cancel. On failure the row stays
+  /// removed in-memory and the cancel is queued for replay — this
+  /// keeps the user's intent visible across network loss instead of
+  /// restoring a row they explicitly asked to remove.
   Future<void> cancel(String id) async {
     final api = await ref.read(requestsApiProvider.future);
     final current = state.value ?? const <AdminRequest>[];
     state = AsyncData(current.where((r) => r.id != id).toList());
     try {
       await api.cancel(id);
-    } catch (e, st) {
-      state = AsyncData(current);
-      Error.throwWithStackTrace(e, st);
+    } catch (_) {
+      await ref
+          .read(mutationQueueProvider)
+          .enqueue(MutationKinds.requestCancel, {'id': id});
     }
   }
 }

flutter_client/lib/shared/widgets/track_actions/track_actions_sheet.dart

@@ -1,8 +1,12 @@
+import 'package:drift/drift.dart' as drift;
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../../../api/endpoints/likes.dart';
+import '../../../cache/audio_cache_manager.dart' show appDbProvider;
+import '../../../cache/db.dart';
+import '../../../cache/mutation_queue.dart';
 import '../../../likes/likes_provider.dart';
 import '../../../models/track.dart';
 import '../../../player/player_provider.dart';
@@ -255,7 +259,38 @@ typedef AddToPlaylistAction = Future<void> Function(String playlistId, String tr
 
 final addToPlaylistActionProvider = Provider<AddToPlaylistAction>((ref) {
   return (playlistId, trackId) async {
-    final api = await ref.read(playlistsApiProvider.future);
-    await api.appendTracks(playlistId, [trackId]);
+    final db = ref.read(appDbProvider);
+    // Optimistic drift write so the playlist detail screen shows the
+    // new track instantly. The position is best-effort — append after
+    // the current max for this playlist. The server's authoritative
+    // position lands on next playlistDetailProvider fetch (SWR), or
+    // when the queued mutation replays.
+    final maxPos = await (db.selectOnly(db.cachedPlaylistTracks)
+          ..addColumns([db.cachedPlaylistTracks.position.max()])
+          ..where(db.cachedPlaylistTracks.playlistId.equals(playlistId)))
+        .getSingleOrNull();
+    final nextPos =
+        ((maxPos?.read(db.cachedPlaylistTracks.position.max()) ?? -1) + 1);
+    await db.into(db.cachedPlaylistTracks).insertOnConflictUpdate(
+          CachedPlaylistTracksCompanion.insert(
+            playlistId: playlistId,
+            trackId: trackId,
+            position: drift.Value(nextPos),
+          ),
+        );
+
+    try {
+      final api = await ref.read(playlistsApiProvider.future);
+      await api.appendTracks(playlistId, [trackId]);
+    } catch (_) {
+      // REST failed — keep the optimistic drift row; queue the call.
+      await ref.read(mutationQueueProvider).enqueue(
+        MutationKinds.playlistAppend,
+        {
+          'playlistId': playlistId,
+          'trackIds': [trackId],
+        },
+      );
+    }
   };
 });

flutter_client/test/quarantine/quarantine_provider_test.dart

@@ -92,8 +92,8 @@ void main() {
     expect(rows.first.reason, 'bad_rip');
   });
 
-  test('flag rolls back the drift insert on server failure', skip: _skipDrift,
-      () async {
+  test('flag keeps drift optimistic + queues mutation on server failure',
+      skip: _skipDrift, () async {
     final db = AppDb(NativeDatabase.memory());
     addTearDown(db.close);
     final api = _StubQuarantineApi(shouldThrow: true);
@@ -101,14 +101,18 @@ void main() {
     addTearDown(container.dispose);
 
     await container.read(myQuarantineProvider.future);
-    await expectLater(
-      () => container
-          .read(myQuarantineProvider.notifier)
-          .flag(_track, 'bad_rip', ''),
-      throwsException,
-    );
+    // No longer rethrows — flag swallows the REST failure and queues
+    // for replay so the user's intent persists offline.
+    await container
+        .read(myQuarantineProvider.notifier)
+        .flag(_track, 'bad_rip', '');
     final rows = await db.select(db.cachedQuarantineMine).get();
-    expect(rows, isEmpty);
+    expect(rows, hasLength(1),
+        reason: 'optimistic drift row should persist across REST failure');
+    final mutations = await db.select(db.cachedMutations).get();
+    expect(mutations, hasLength(1),
+        reason: 'failed flag should have been enqueued for replay');
+    expect(mutations.first.kind, 'quarantine.flag');
   });
 
   test('unflag deletes the drift row and calls the server', skip: _skipDrift,