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

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

test/library/playlist_service_test.dart

@@ -0,0 +1,42 @@
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/library/database.dart';
+import 'package:melo/library/playlist_service.dart';
+
+void main() {
+  late MeloDb db;
+  late PlaylistService service;
+
+  setUp(() {
+    db = MeloDb(NativeDatabase.memory());
+    service = PlaylistService(db);
+  });
+
+  tearDown(() async => db.close());
+
+  test('createPlaylist delegates to db and notifies listeners', () async {
+    var notified = false;
+    service.addListener(() => notified = true);
+
+    await service.createPlaylist('Road Trip');
+
+    final playlists = await db.watchPlaylists().first;
+    expect(playlists.length, 1);
+    expect(playlists.first.name, 'Road Trip');
+    expect(notified, true);
+  });
+
+  test('toggleFavorite delegates to db and notifies listeners', () async {
+    await db.into(db.songs).insert(SongsCompanion.insert(
+      id: 'song-1', path: '/a.mp3', title: 'A', dateAddedMs: 0, updatedAtMs: 0,
+    ));
+
+    var notified = false;
+    service.addListener(() => notified = true);
+
+    await service.toggleFavorite('song-1');
+
+    expect(await db.watchIsFavorite('song-1').first, true);
+    expect(notified, true);
+  });
+}