Problem: `codeforces.py` used `curl_cffi` to bypass Cloudflare when
fetching contest problem HTML, making it unavailable in the nix python
env and requiring an extra dependency.
Solution: rewrite `_fetch_problems_html` to use scrapling
`StealthySession` with `solve_cloudflare=True`, matching the existing
CF submit pattern. Extend `needs_browser` in `scraper.lua` to route CF
`metadata` and `tests` through the FHS env on NixOS. Remove `curl-cffi`
from `pyproject.toml`, `flake.nix`, and test mocks.
