fix: replace curl_cffi with scrapling in codeforces metadata (#334)

## 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 across `pyproject.toml` and `flake.nix`.

## 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.

flake.nix

@@ -19,7 +19,6 @@
         pkgs.python312.withPackages (ps: [
           ps.backoff
           ps.beautifulsoup4
-          ps.curl-cffi
           ps.httpx
           ps.ndjson
           ps.pydantic
@@ -31,7 +30,6 @@
         pkgs.python312.withPackages (ps: [
           ps.backoff
           ps.beautifulsoup4
-          ps.curl-cffi
           ps.httpx
           ps.ndjson
           ps.pydantic