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.
## Problem
`_submit_sync` was a 170-line nested closure with `_solve_turnstile` and
the browser-install block further nested inside it. Status events went
to
stderr, which `run_scraper()` silently discards, leaving the user with a
10–30s silent hang after credential entry. The NDJSON spawn path also
lacked stdin support, so submit had no streaming path at all.
## Solution
Extract `_TURNSTILE_JS`, `_solve_turnstile`, `_ensure_browser`, and
`_submit_headless` to module level in `atcoder.py`; status events
(`installing_browser`, `checking_login`, `logging_in`, `submitting`) now
print to stdout as NDJSON. Add stdin pipe support to the NDJSON spawn
path in `scraper.lua` and switch `M.submit` to streaming with an
`on_status` callback. Wire `on_status` in `submit.lua` to fire
`vim.notify` for each phase transition.
Problem: lua-language-server is not available in the dev shell, making
it impossible to run local type-checking diagnostics.
Solution: add lua-language-server to the devShell packages.
