fix(voice): resolve Redis subscriber corruption, transmitter races, and DB error logging

Redis root cause: VoiceHandler passed redisPub to transmitter.start(),
which called .subscribe() on it — permanently converting the publish
connection to subscriber mode. Every subsequent command reply and status
update failed.

Fixes:
1. Transmitter now creates its own Redis client via new IORedis()
2. Mutual exclusion gate serialises start/stop to prevent null-deref races
3. PassThrough drain listeners cleaned up to stop MaxListenersExceeded
4. FFmpeg SIGTERM flagged as expected exit (no more false level-50 errors)
5. Voice recording repo captures PG error code/detail for diagnostics

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

services/discord-gateway/src/modules/command-handler/commandHandler.ts

@@ -81,7 +81,6 @@ export class CommandHandler {
     this.voiceHandler = new VoiceHandler(
       client,
       voiceController,
-      this.redisPub,
     );
     this.mediaHandler = new MediaHandler();
     this.guildHandler = new GuildHandler(client);