Support raw 16 bit PCM data

in `def get_audio_from_websocket(self, websocket):` we can call

```
audio_np = np.frombuffer(audio_data, dtype=np.int16)
# Normalize to float32 range [-1.0, 1.0]
return audio_np.astype(np.float32) / 32768.0
```

if flag set on server to specify raw PCM input.