Draft: feat: add FastAPI backend
- FastAPI + uvicorn with routers for users, batch, leaderboard, roster, weekly-performance, corpus
- GitLab REST + GraphQL clients with SSL/timeout config
- Neon PostgreSQL integration via SQLAlchemy
- SSE streaming for batch analytics
- Docker + docker-compose setup
- Separate .env, .gitignore, .gitlab-ci.yml, README for backend