feat(database): add Playlists, Favorites, PlaybackHistory tables

- Playlists + PlaylistSongs tables with position-ordered join query
- Favorites table with toggle + watch (reactive heart icon support)
- PlaybackHistory table for resume-position tracking
- Schema v2 with migration from v1
- Follows existing Songs/Folders sync-ready pattern (uuid id, updatedAtMs, deleted)

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

test/library/database_playlists_test.dart

@@ -0,0 +1,110 @@
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/library/database.dart';
+
+void main() {
+  late MeloDb db;
+
+  setUp(() {
+    db = MeloDb(NativeDatabase.memory());
+  });
+
+  tearDown(() async {
+    await db.close();
+  });
+
+  group('Playlists', () {
+    test('createPlaylist inserts and watchPlaylists emits it', () async {
+      await db.createPlaylist('Workout');
+      final playlists = await db.watchPlaylists().first;
+      expect(playlists.length, 1);
+      expect(playlists.first.name, 'Workout');
+    });
+
+    test('deletePlaylist removes it', () async {
+      final id = await db.createPlaylist('Temp');
+      await db.deletePlaylist(id);
+      final playlists = await db.watchPlaylists().first;
+      expect(playlists, isEmpty);
+    });
+  });
+
+  group('PlaylistSongs', () {
+    test('addSongToPlaylist + watchPlaylistSongs returns song in order', () async {
+      final playlistId = await db.createPlaylist('My List');
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-1',
+        path: '/a.mp3',
+        title: 'Song A',
+        dateAddedMs: 0,
+        updatedAtMs: 0,
+      ));
+
+      await db.addSongToPlaylist(playlistId, 'song-1', 0);
+      final songs = await db.watchPlaylistSongs(playlistId).first;
+      expect(songs.length, 1);
+      expect(songs.first.title, 'Song A');
+    });
+
+    test('removeSongFromPlaylist removes it', () async {
+      final playlistId = await db.createPlaylist('My List');
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-1',
+        path: '/a.mp3',
+        title: 'Song A',
+        dateAddedMs: 0,
+        updatedAtMs: 0,
+      ));
+      await db.addSongToPlaylist(playlistId, 'song-1', 0);
+      await db.removeSongFromPlaylist(playlistId, 'song-1');
+      final songs = await db.watchPlaylistSongs(playlistId).first;
+      expect(songs, isEmpty);
+    });
+  });
+
+  group('Favorites', () {
+    test('toggleFavorite marks then unmarks a song', () async {
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-1',
+        path: '/a.mp3',
+        title: 'Song A',
+        dateAddedMs: 0,
+        updatedAtMs: 0,
+      ));
+
+      await db.toggleFavorite('song-1');
+      expect(await db.watchIsFavorite('song-1').first, true);
+
+      await db.toggleFavorite('song-1');
+      expect(await db.watchIsFavorite('song-1').first, false);
+    });
+
+    test('watchFavorites returns favorited songs only', () async {
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-1', path: '/a.mp3', title: 'Fav', dateAddedMs: 0, updatedAtMs: 0,
+      ));
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-2', path: '/b.mp3', title: 'NotFav', dateAddedMs: 0, updatedAtMs: 0,
+      ));
+      await db.toggleFavorite('song-1');
+
+      final favs = await db.watchFavorites().first;
+      expect(favs.length, 1);
+      expect(favs.first.title, 'Fav');
+    });
+  });
+
+  group('PlaybackHistory', () {
+    test('recordPlayback + lastPosition roundtrip', () async {
+      await db.into(db.songs).insert(SongsCompanion.insert(
+        id: 'song-1', path: '/a.mp3', title: 'A', dateAddedMs: 0, updatedAtMs: 0,
+      ));
+      await db.recordPlayback('song-1', 45000);
+      expect(await db.lastPosition('song-1'), 45000);
+    });
+
+    test('lastPosition returns null when never played', () async {
+      expect(await db.lastPosition('unknown'), isNull);
+    });
+  });
+}