Add TTS Engine

Goal

Integrate a new text-to-speech engine into Voicebox end-to-end: dependency research, backend protocol implementation, frontend UI wiring, PyInstaller bundling, and frozen-build verification. The user should only need to test the final build locally.

Reference Doc

The full phased guide lives at docs/content/docs/developer/tts-engines.mdx. Read this file in its entirety before starting. It contains:

• Phase 0: Dependency research (mandatory before writing code)
• Phase 1: Backend implementation (TTSBackend protocol)
• Phase 2: Route and service integration (usually zero changes)
• Phase 3: Frontend integration (5 files)
• Phase 4: Dependencies (requirements.txt, justfile, CI, Docker)
• Phase 5: PyInstaller bundling (build_binary.py + server.py)
• Phase 6: Common upstream workarounds
• Implementation checklist (gate between phases)