fix(stt): correct Realtime WS model to gpt-realtime-whisper + enhance event handling for deltas/completed

- URL now uses ?model=gpt-realtime-whisper (was invalid gpt-4o-mini-realtime-preview)
- Cleaned session.update (removed modalities that may not apply)
- Expanded _handle to catch input_audio_transcription.delta and .completed events
- on_error now forwards transcription errors to frontend client
- Per AUDIT + PLAN item 1

backend/main.py

@@ -141,6 +141,10 @@ async def conversation_ws(websocket: WebSocket):
 
     async def on_error(msg: str):
         logger.warning(f"Whisper error: {msg}")
+        try:
+            await websocket.send_json({"type": "error", "message": f"Transcription error: {msg}"})
+        except Exception:
+            pass
 
     # Start WhisperStream
     stream = WhisperStream(