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/app/observability.py

@@ -204,3 +204,20 @@ def save_voice(trace_id: str | None, message_id: int, content: bytes,
     except Exception:
         logger.warning("obs: save_voice failed for trace %s", trace_id, exc_info=True)
         return None
+
+
+def save_enhanced_voice(trace_id: str | None, message_id: int, content: bytes,
+                        ext: str = ".ogg") -> str | None:
+    """Persist an enhanced voice note under OBS_VOICE_DIR/{YYYYMMDD}/{trace_id}_{message_id}_enhanced{ext}."""
+    try:
+        day = datetime.utcnow().strftime("%Y%m%d")
+        base = Path(settings.OBS_VOICE_DIR) / day
+        base.mkdir(parents=True, exist_ok=True)
+        if ext and not ext.startswith("."):
+            ext = f".{ext}"
+        path = base / f"{trace_id}_{message_id}_enhanced{ext or '.ogg'}"
+        path.write_bytes(content)
+        return str(path)
+    except Exception:
+        logger.warning("obs: save_enhanced_voice failed for trace %s", trace_id, exc_info=True)
+        return None