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>
