Add explicit reference text config and clean default reference ...

- Add DEFAULT_REF_TEXT setting to skip Whisper transcription

- Pass ref_text through F5 backend and warm-up path

- Document new env variables and warm-up usage

- Update latency notes with explicit ref_text timing

Initial voice TTS server with F5-TTS backend, WebSocket streaming and warm-up ...

- FastAPI WebSocket API for real-time chunked TTS

- F5-TTS GPU backend with reference caching and auto-transcription

- Dummy backend for offline tests

- Streaming text segmenter, session state, stop/resume

- Startup warm-up, PCM16/base64 audio, basic tests

- Documentation: architecture, protocol, usage, roadmap, technical notes