- Add audio_enhancer module with 60s timeout, zero-transcoding default, and fail-soft fallback
- Persist enhanced audio in daily folders with _enhanced suffix and emit audio.enhance.result event
- Connect audio enhancer to transcribe_multi and transcribe in stt.py
- Forward message_id and trace_id from handlers
- Ensure consensus_text fallback persistence and remove scenario leakage in nlu.py
- Add test_enhancer.py unit suite and update smoke_test.py
