feat: add streaming STT service with Whisper backend

- stt_streaming.py: HTTP-based STT using external Whisper service
- stt_streaming_local.py: ROCm-based local Whisper inference
- Voice Activity Detection (VAD) with WebRTC
- Interrupt detection for barge-in support
- Session state management (listening/responding)
- OpenTelemetry instrumentation with HyperDX support
- Dockerfile variants for HTTP and ROCm deployments

.gitignore

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+.venv/
+__pycache__/
+*.pyc
+*.pyo
+.pytest_cache/
+.mypy_cache/
+*.egg-info/
+dist/
+build/
+.env
+.env.local