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();
+  }
+}