fix: auto-play next song — set ffmpeg=null on close, handle async playNext

Two bugs prevented auto-advance:
1. FFmpeg close handler was empty — ChildProcess object persisted after
   close, so frame loop check `!this.ffmpeg` never triggered trackEnd
2. playNext() is async (resolves URL) but trackEnd handler didn't catch
   rejected promises, causing silent failures

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

src/audio/player.ts

@@ -79,7 +79,7 @@ export class AudioPlayer extends EventEmitter {
     });
 
     this.ffmpeg.on("close", () => {
-      // Let frame loop drain remaining buffer
+      this.ffmpeg = null; // Signal frame loop that no more data is coming
     });
 
     this.ffmpeg.on("error", (err) => {

src/bot/instance.ts

@@ -80,12 +80,16 @@ export class BotInstance extends EventEmitter {
 
     this.player.on("trackEnd", () => {
       this.logger.debug("Track ended, advancing queue");
-      this.playNext();
+      this.playNext().catch((err) => {
+        this.logger.error({ err }, "playNext failed after trackEnd");
+      });
     });
 
     this.player.on("error", (err: Error) => {
       this.logger.error({ err }, "Player error");
-      this.playNext();
+      this.playNext().catch((err2) => {
+        this.logger.error({ err: err2 }, "playNext failed after player error");
+      });
     });
   }