Problem: each scraper defined its own timeout constants
(`TIMEOUT_S`, `TIMEOUT_SECONDS`) with inconsistent values (15s vs 30s)
and browser timeouts were scattered as magic numbers (60000, 15000,
5000, 500).
Solution: introduce `scrapers/timeouts.py` with named constants for
HTTP requests, browser session/navigation/element/turnstile/settle
timeouts, and submission polling. All six scrapers now import from
the shared module.
Problem: lua typecheck flagged missing start_time field on ContestSummary;
ty flagged BeautifulSoup Tag/NavigableString union on csrf_input.get(),
a 3-tuple unpack where _extract_problem_info now returns 4 values in
cses.py, and an untyped list assignment in usaco.py.
Solution: add start_time? to ContestSummary LuaDoc, guard csrf_input
with hasattr check and type: ignore, unpack precision from
_extract_problem_info in cses.py callers, and use cast() in usaco.py.
Add KattisScraper and USACOScraper with contest list, metadata, and
test case fetching. Register kattis and usaco in PLATFORMS,
PLATFORM_DISPLAY_NAMES, and default platform configs.
