feat(metadata): 元数据补全后主动推送界面刷新

之前标签补全后，收藏/文件列表要退出重进才显示新信息。
- 新增 MetadataEvents 事件总线（MetadataUpdate: path + songs 行）
- MetadataService 写库成功后 publish
- 首页收藏列表订阅：按 path 替换对应 SongItem（标题/艺术家/封面）
- WebDAV 文件列表订阅：按 path 更新 _metaByPath

效果：歌曲播完标签抓取完成后，列表里的那一行自动变成标签信息，
不需要返回重进。

lib/pages/home_page.dart

@@ -1,6 +1,8 @@
+import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../services/audio_service.dart';
+import '../services/metadata_events.dart';
 import '../services/webdav_service.dart';
 import '../widgets/magnetic_scroll_physics.dart';
 import 'webdav_setup_page.dart';
@@ -163,6 +165,7 @@ class _HomePageState extends BaseState<HomePage> {
   /// 0 = 我的收藏，1 = 歌单列表（左右滑动切换）
   int _homeTab = 0;
   final PageController _homePageController = PageController();
+  StreamSubscription<MetadataUpdate>? _metaSub;
   List<PlaylistSummary> _playlistSummaries = [];
 
   // ========== 滚动控制 ==========
@@ -240,6 +243,31 @@ class _HomePageState extends BaseState<HomePage> {
     }
   }
 
+  /// 元数据补全后，按 path 更新收藏列表里的对应行
+  void _onMetadataUpdate(MetadataUpdate update) {
+    final index = _favorites.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 = _favorites[index];
+    safeSetState(() {
+      _favorites[index] = SongItem(
+        path: old.path,
+        fileName: old.fileName,
+        title: title.isNotEmpty ? title : old.title,
+        artist: artist.isNotEmpty ? artist : old.artist,
+        artworkPath: artwork.isNotEmpty ? artwork : old.artworkPath,
+        sourceTag: old.sourceTag,
+        metadataState: (title.isNotEmpty || old.title != null)
+            ? 'success'
+            : 'unknown',
+      );
+    });
+  }
   void _onScroll() {
     if (mounted) safeSetState(() {});
   }