feat(services): add PlaylistService, wire into app providers

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/library/playlist_service.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/foundation.dart';
+import 'database.dart';
+
+/// Koordiniert Playlisten- und Favoriten-Operationen; benachrichtigt Listener
+/// nach jeder Mutation (für Feedback wie SnackBars — die Listen selbst
+/// beobachten UIs direkt über die watch()-Streams von [MeloDb]).
+class PlaylistService extends ChangeNotifier {
+  PlaylistService(this.db);
+  final MeloDb db;
+
+  Future<String> createPlaylist(String name, {String? description}) async {
+    final id = await db.createPlaylist(name, description: description);
+    notifyListeners();
+    return id;
+  }
+
+  Future<void> deletePlaylist(String id) async {
+    await db.deletePlaylist(id);
+    notifyListeners();
+  }
+
+  Future<void> addSongToPlaylist(String playlistId, String songId, int position) async {
+    await db.addSongToPlaylist(playlistId, songId, position);
+    notifyListeners();
+  }
+
+  Future<void> removeSongFromPlaylist(String playlistId, String songId) async {
+    await db.removeSongFromPlaylist(playlistId, songId);
+    notifyListeners();
+  }
+
+  Future<void> reorderSong(String playlistId, String songId, int newPosition) async {
+    await db.reorderPlaylistSong(playlistId, songId, newPosition);
+    notifyListeners();
+  }
+
+  Future<void> toggleFavorite(String songId) async {
+    await db.toggleFavorite(songId);
+    notifyListeners();
+  }
+}

lib/main.dart

@@ -6,6 +6,7 @@ import 'home_screen.dart';
 import 'library/database.dart';
 import 'library/library_screen.dart';
 import 'library/library_service.dart';
+import 'library/playlist_service.dart';
 import 'library/search_screen.dart';
 import 'player/audio_handler.dart';
 import 'player/mini_player.dart';
@@ -14,11 +15,13 @@ import 'shared/theme.dart';
 late final MeloAudioHandler _handler;
 late final MeloDb _db;
 late final LibraryService _library;
+late final PlaylistService _playlists;
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
   _db = MeloDb();
   _library = LibraryService(_db);
+  _playlists = PlaylistService(_db);
   _handler = await AudioService.init(
     builder: () => MeloAudioHandler(),
     config: const AudioServiceConfig(
@@ -40,6 +43,7 @@ class MeloApp extends StatelessWidget {
         Provider<MeloAudioHandler>.value(value: _handler),
         Provider<MeloDb>.value(value: _db),
         ChangeNotifierProvider<LibraryService>.value(value: _library),
+        ChangeNotifierProvider<PlaylistService>.value(value: _playlists),
       ],
       child: MaterialApp(
         title: 'Melo',