Log more errors

The backend basically logs no errors at all. It also doesn't log anything relating to the websocket which makes debugging impossible.

The backend should log significantly more when it comes to submissions and testing. Even complete failures are not logged at all.

No files or tests are named. Start by tracing the backend paths for websocket communication, submissions, and testing, then identify where failures are currently handled without logging. Done means relevant successful operations and complete failures produce useful logs, with tests or verification covering the added cases.

Written by the indexing model from the issue text.