fix(player): retry-skip in /play-playlist when first song can't resolve

QQ Music sometimes refuses to issue a play URL for individual songs
(upstream returns result=104003, "no copyright/payment"). The
/play-playlist endpoint only called resolveAndPlay once on the first
song, so when that one was 104003 the bot would sit silently with no
feedback — exactly what looked like "播放全部 没有反应".

Make BotInstance.playNext() public and call it as a fallback when the
first resolveAndPlay fails. playNext already has the 3-attempt
retry-skip used by trackEnd auto-advance, so we get the same
graceful skip behavior for explicit playlist plays.

Also tighten the response message so the user can tell when the bot
loaded songs but none were playable.

Single-song /play-song still returns "Cannot play" on failure (no
queue to advance through); UX surfacing of that goes in a follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bot/instance.ts

@@ -734,7 +734,13 @@ export class BotInstance extends EventEmitter {
     ].join("\n");
   }
 
-  private async playNext(): Promise<void> {
+  /**
+   * Advance the queue and play the next song. If the resolved URL fails
+   * (e.g., copyright/region restrictions for QQ), skips up to 3 more
+   * songs looking for a playable one. Public so REST endpoints that
+   * seed the queue can fall back to this retry-skip behavior.
+   */
+  async playNext(): Promise<void> {
     if (this.isAdvancing || !this.connected) return;
     this.isAdvancing = true;
     try {