feat: add cloud TTS switcher

[No1Hoo]

Summary

• Add runtime backend TTS config endpoints for reading/updating active cloud GPU URL.
• Add a frontend 算力 panel for saving Kaggle/Colab ngrok URLs and switching without backend restart.
• Document the new switcher flow in README.
• Add backend tests for TTS config update validation.

Test Plan

• .venv/bin/python -m pytest -q
• .venv/bin/python -m ruff check .
• cd frontend && npm run lint
• cd frontend && npm run build
• Browser automation: open 算力 panel and apply a test URL, then verify /tts-config updates.