TiDB Labs uv reference for LLMs

Scope
- Python dependency management in `backend/` uses `uv`.

Canonical files
- `backend/pyproject.toml`
- `backend/.python-version`

Common commands
- `cd backend && uv sync`
- `cd backend && uv run python manage.py runserver 0.0.0.0:8000`
- `cd backend && uv run celery -A labs worker --concurrency=5 --loglevel=INFO`
- `cd backend && uv run python manage.py generate_lab_list`
- `cd backend && uv run python manage.py test`

Caveats
- Backend validation depends on TiDB, Redis, and the external lab-spec repo at
  `backend/labs/labs`.