fix: stt.yaml env var WHISPER_URL→STT_URL + tts.py improvements

- stt.yaml: rename WHISPER_URL to STT_URL to match what stt.py reads
- tts.py: improve WAV handling (BytesIO fix), sentence splitting, robust
  _read_wav_bytes with wave+soundfile+raw-PCM fallbacks
- Add __pycache__/ to .gitignore

stt.yaml

@@ -28,7 +28,7 @@ spec:
               name: http
               protocol: TCP
           env:
-            - name: WHISPER_URL
+            - name: STT_URL
               # Ray Serve endpoint - routes to /whisper prefix
               value: "http://ai-inference-serve-svc.ai-ml.svc.cluster.local:8000/whisper"
             - name: MLFLOW_TRACKING_URI