Problem: CSES submit was a stub returning "not yet implemented".
Solution: authenticate via web login + API token bridge (POST
`/login` form, then POST `/api/login` and confirm the auth page),
submit source to `/api/courses/problemset/submissions` with
base64-encoded content, and poll for verdict. Uses the same
username/password credential model as AtCoder.
Also update `scraper.lua` to pass the full ndjson event to `on_status`
(instead of just the status string) and handle `credentials` events
for platforms that return updated credentials.
Barrett Ruth
972044fd0f