fix: resolve critical race conditions and high-severity bugs

- Add isAdvancing guard to playNext() to prevent concurrent calls from
  skipping songs (trackEnd + user !next race condition)
- Replace removeAllListeners in WebSocket setup with tracked named
  listeners, attach only once per bot instead of every 5 seconds
- Add .catch() to unhandled playNext() in cmdVote
- Add sessionId counter to AudioPlayer to discard stale setTimeout
  callbacks after stop()+play() transitions
- Defer nulling TS3 client until disconnect() promise resolves
- Add null checks for providers in play-by-id and add-by-id endpoints
- Add PCM buffer backpressure (pause FFmpeg at 960KB, resume at 480KB)
- Change volume slider from @input to @change to avoid flooding server
- Wrap error-reporting sendTextMessage in try/catch to prevent
  unhandled rejection in error handler
- Clamp elapsed getter to song duration
- Log FFmpeg stderr at debug level instead of swallowing
- Return null from prev() in Sequential mode instead of wrapping

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

src/ts-protocol/client.ts

@@ -39,6 +39,7 @@ export class TS3Client extends EventEmitter {
   private identity: Identity;
   private clientId = 0;
   private logger: Logger;
+  private disconnecting = false;
 
   constructor(private options: TS3ClientOptions, logger: Logger) {
     super();
@@ -167,9 +168,15 @@ export class TS3Client extends EventEmitter {
   }
 
   disconnect(): void {
-    if (this.client) {
-      this.client.disconnect().catch(() => {});
-      this.client = null;
+    if (this.client && !this.disconnecting) {
+      this.disconnecting = true;
+      const client = this.client;
+      client.disconnect().catch(() => {}).finally(() => {
+        if (this.client === client) {
+          this.client = null;
+        }
+        this.disconnecting = false;
+      });
     }
     this.clientId = 0;
     this.logger.info("Disconnected from TeamSpeak server");