refactor: lazy URL resolution — store only metadata, fetch URL at play time

Follows TS3AudioBot-NetEaseCloudmusic-plugin pattern:
- QueuedSong.url is now optional (metadata-only queue entries)
- resolveAndPlay() fetches URL on-demand right before playback
- Playlist/album/FM load instantly (only metadata stored)
- playNext() resolves URL lazily, skips up to 3 songs on failure
- Avoids expired URLs for songs deep in the queue

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/audio/queue.ts

@@ -11,7 +11,7 @@ export interface QueuedSong {
   artist: string;
   album: string;
   platform: "netease" | "qq";
-  url: string;
+  url?: string; // resolved lazily at play time
   coverUrl: string;
   duration: number; // seconds
 }