feat: P7 在线播放（播放队列、媒体控制）与自建歌单

- 播放器：统一 PlayItem 队列，顺序 / 列表循环 / 单曲循环 / 随机，滚动歌词含翻译，Media Session，队列与进度存 localStorage
- 在线播放音质可在设置中选择；曲库已有的歌优先播放本地文件
- 自建歌单：混排在线曲目与本地文件，拖动排序，一键下载曲库缺失曲目

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/db.ts

@@ -50,6 +50,31 @@ db.exec(`
     updated_at     TEXT NOT NULL
   );
   CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status, id);
+  CREATE TABLE IF NOT EXISTS playlists (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    name        TEXT NOT NULL,
+    description TEXT NOT NULL DEFAULT '',
+    created_at  TEXT NOT NULL,
+    updated_at  TEXT NOT NULL
+  );
+  CREATE TABLE IF NOT EXISTS playlist_items (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    playlist_id INTEGER NOT NULL REFERENCES playlists(id) ON DELETE CASCADE,
+    position    INTEGER NOT NULL,
+    item_key    TEXT NOT NULL,
+    source      TEXT,
+    track_id    TEXT,
+    path        TEXT,
+    name        TEXT NOT NULL,
+    artists     TEXT NOT NULL DEFAULT '',
+    album       TEXT NOT NULL DEFAULT '',
+    album_id    TEXT,
+    cover       TEXT,
+    duration    INTEGER,
+    added_at    TEXT NOT NULL,
+    UNIQUE (playlist_id, item_key)
+  );
+  CREATE INDEX IF NOT EXISTS idx_playlist_items_pos ON playlist_items(playlist_id, position);
 `);
 
 export interface HistoryEntry {