feat(playlist): 歌单变更与歌单内元数据实时刷新

两个边界场景：
1) 新建/重命名/删除歌单、加入/移除歌曲后，主页歌单页不刷新
2) 歌单详情页里歌曲补全元数据后，标题/封面不更新

- 新增 PlaylistEvents（void 事件）
- PlaylistRepository 的创建/重命名/删除/批量加入/移除后 publish
- 主页订阅后重新拉取歌单摘要
- 歌单详情页订阅 MetadataEvents，按 path 就地更新行

lib/pages/playlist_detail_page.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 // lib/pages/playlist_detail_page.dart
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
@@ -5,6 +6,7 @@ import 'package:provider/provider.dart';
 import '../database/song_database.dart';
 import '../repositories/playlist_repository.dart';
 import '../services/audio_service.dart';
+import '../services/metadata_events.dart';
 import '../widgets/song_artwork.dart';
 
 /// 歌单详情：展示歌单内歌曲，支持播放全部 / 单曲播放 / 移除
@@ -41,14 +43,40 @@ class _PlaylistDetailPageState extends State<PlaylistDetailPage> {
   final SongDatabase _db = SongDatabase();
 
   List<_SongRow> _songs = [];
+  StreamSubscription<MetadataUpdate>? _metaSub;
   bool _loading = true;
 
   @override
   void initState() {
     super.initState();
+    _metaSub = MetadataEvents().stream.listen(_onMetadataUpdate);
     _load();
   }
 
+  @override
+  void dispose() {
+    _metaSub?.cancel();
+    super.dispose();
+  }
+
+  /// 歌单里的歌补全元数据后，就地更新对应行
+  void _onMetadataUpdate(MetadataUpdate update) {
+    final index = _songs.indexWhere((item) => item.path == update.path);
+    if (index < 0 || !mounted) return;
+    final row = update.row;
+    final title = (row['title'] as String?)?.trim() ?? '';
+    final artist = (row['artist'] as String?)?.trim() ?? '';
+    final artwork = (row['artwork_path'] as String?)?.trim() ?? '';
+    final old = _songs[index];
+    setState(() {
+      _songs[index] = _SongRow(
+        path: old.path,
+        title: title.isNotEmpty ? title : old.title,
+        artist: artist.isNotEmpty ? artist : old.artist,
+        artworkPath: artwork.isNotEmpty ? artwork : old.artworkPath,
+      );
+    });
+  }
   Future<void> _load() async {
     setState(() => _loading = true);
     try {