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/config.py

@@ -73,6 +73,15 @@ class Settings(BaseSettings):
     STT_MODEL: str = "whisper-1"
     STT_LANGUAGE: str = "fa"
 
+    # Audio Enhancement / Preprocessing (denoising & speech quality enhancement)
+    AUDIO_ENHANCE_ENABLED: bool = False
+    AUDIO_ENHANCE_BASE_URL: str = ""
+    AUDIO_ENHANCE_API_KEY: str = ""
+    AUDIO_ENHANCE_MODEL: str = ""
+    AUDIO_ENHANCE_TIMEOUT_SECONDS: float = 60.0
+    AUDIO_ENHANCE_TARGET_FORMAT: str = ""  # empty = pass raw bytes directly; e.g. "wav" / "mp3"
+    AUDIO_ENHANCE_METHOD: str = "auto"     # "auto" | "raw_stream" | "multipart"
+
     @property
     def stt_base_url(self) -> str:
         return self.STT_BASE_URL or self.AI_BASE_URL