fix: submission_status_cleanup recovers all non-terminal states

[hanane-ca]

Fixes #2413

Problem

submission_status_cleanup() only filters status=Running, so submissions stuck in Submitted, Preparing, or Scoring are never recovered by the periodic cleanup. They hang indefinitely and consume quota permanently.

The existing started_when check also fails for submissions that never reached Running (null started_when).

Changes

• src/apps/competitions/tasks.py: extend filter from status=Running to status__in=[Submitted, Preparing, Running, Scoring]
• Use created_when as fallback reference time when started_when is null
• src/apps/competitions/tests/test_submissions.py: add 4 unit tests for the new cleanup coverage + 1 negative test ensuring recent submissions are not prematurely cleaned

Testing

All 6 tests pass:

$ docker compose exec django python manage.py test competitions.tests.test_submissions.TestSubmissionTasks --verbosity=2

test_cancelling_parent_submission_cancels_all_children ... ok
test_cleanup_does_not_touch_recent_non_terminal_submissions ... ok
test_cleanup_recovers_stuck_preparing_submissions ... ok
test_cleanup_recovers_stuck_scoring_submissions ... ok
test_cleanup_recovers_stuck_submitted_submissions ... ok
test_submissions_are_cancelled_if_running_24_hours_past_execution_time_limit ... ok

Ran 6 tests in 2.231s
OK