After bot leave/rejoin, Discord doesn't resend SPEAKING events for
users already in the channel. This left SSRC unmapped and all audio
was silently dropped by DAVE decrypt.
Fixes:
- Skip DAVE for unknown SSRCs instead of dropping (passthrough)
- Handle "UnencryptedWhenPassthroughDisabled" DAVE errors gracefully
- Auto-infer user_id from sole allowed member in voice channel
- Pass allowed_user_ids to VoiceReceiver for secure inference