Problem: scrape_contest_list paginated the entire Kattis problem database
(3000+ problems) treating each as a "contest". scrape_contest_metadata
only handled single-problem access. stream_tests_for_category_async could
not fetch tests for multiple problems in a real contest.
Solution: replace the paginated problem loop with a single GET to
/contests that returns ~150 real timed contests. Add contest-aware path
to scrape_contest_metadata that fetches /contests/{id}/problems and
returns all problem slugs; fall back to single-problem path when the ID
is not a contest. Add _stream_single_problem helper and update
stream_tests_for_category_async to fan out concurrently over all contest
problem slugs before falling back to the single-problem path.
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.
