feat(audio): add pluggable audio preprocessing and denoising layer before STT

- 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

bot/tools/smoke_test.py

@@ -255,6 +255,17 @@ def _install_fake_stores() -> None:
 
     runtime.LinkStore.get, runtime.LinkStore.upsert, runtime.LinkStore.delete = link_get, link_upsert, link_delete
 
+    import app.observability as obs
+    async def fake_obs(*args, **kwargs):
+        pass
+    async def fake_obs_trace(*args, **kwargs):
+        return None
+    obs.event = fake_obs
+    obs.trace_start = fake_obs
+    obs.link_draft = fake_obs
+    obs.set_trace_status = fake_obs
+    obs.latest_trace_for_draft = fake_obs_trace
+
 
 async def fake_log(self, chat_id, direction, kind, body=None):
     pass
@@ -263,16 +274,16 @@ async def fake_log(self, chat_id, direction, kind, body=None):
 NLU_QUEUE: list[dict] = []
 
 
-async def fake_understand(settings, text, draft_summary=None):
+async def fake_understand(settings, text, draft_summary=None, *args, **kwargs):
     return NLU_QUEUE.pop(0) if NLU_QUEUE else {"intent": "unknown", "patch": {}, "confidence": 0}
 
 
-async def fake_transcribe(settings, audio, mime_type=None, filename=None):
+async def fake_transcribe(settings, audio, mime_type=None, filename=None, *args, **kwargs):
     return ("سازان نو برای پروژه برج آسمان ۵۰۰ کیلو میلگرد A3 سایز ۱۴ هر کیلو ۴۵ هزار تومن "
             "پرداخت سه فاز")
 
 
-async def fake_transcribe_multi(settings, audio, mime_type=None, filename=None):
+async def fake_transcribe_multi(settings, audio, mime_type=None, filename=None, *args, **kwargs):
     return [{"name": "fake", "text": "سازان نو برای پروژه برج آسمان ۵۰۰ کیلو میلگرد A3 سایز ۱۴ هر کیلو ۴۵ هزار تومن پرداخت سه فاز"}]