fix: replace curl_cffi with scrapling in codeforces metadata

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.

lua/cp/scraper.lua

@@ -45,6 +45,7 @@ local function run_scraper(platform, subcommand, args, opts)
   end
 
   local needs_browser = subcommand == 'submit' or subcommand == 'login'
+    or (platform == 'codeforces' and (subcommand == 'metadata' or subcommand == 'tests'))
 
   if needs_browser then
     utils.setup_nix_submit_env()