fix: typo WISPER_MODEL -> WHISPER_MODEL in audio.py

Typo caused NameError during Whisper transcription.
Fixed by correcting variable name reference.

Co-Authored-By: Claude <noreply@anthropic.com>

src/tools/audio.py

@@ -123,7 +123,7 @@ def transcribe_audio(file_path: str) -> Dict[str, Any]:
         if _MODEL is None:
             logger.info(f"Loading Whisper model: {WHISPER_MODEL}")
             device = "cuda" if ZERO_GPU_AVAILABLE else "cpu"
-            _MODEL = whisper.load_model(WISPER_MODEL, device=device)
+            _MODEL = whisper.load_model(WHISPER_MODEL, device=device)
             logger.info(f"Whisper model loaded on {device}")
 
         # Transcribe audio