fix(compute_worker): make submission lifecycle idempotent against bro…

[AybH26]

@ mention of reviewers

A brief description of the purpose of the changes contained in this PR

compute_worker uses task_acks_late = True, so any worker that dies before acking causes RabbitMQ to redeliver the run payload. The redelivered worker used to re-execute the full lifecycle, causing:

1. scoring_worker_hostname overwritten by the second worker
2. Submission status flipped backwards through Running → Scoring → Finished again
3. upload_submission_scores inserted duplicate SubmissionScore rows, crashing calculate_scores() with MultipleObjectsReturned

This PR makes the lifecycle idempotent on both sides (worker short-circuits + server refuses terminal mutations). Defense-in-depth, Stripe-style.

Issues this PR resolves

Closes #2433

A checklist for hand testing

• Create a submission, let it finish scoring
• Manually call POST /api/submissions/<id>/upload_submission_scores/ with new hostname & scores
• Expected: hostname & scores remain unchanged; worker_attempt_count increments; status stays terminal
• Verify no MultipleObjectsReturned exception in logs
• Check admin shell: Submission.objects.get(pk=26).worker_attempt_count >= 2
• Confirm no duplicate score rows for same column

Any relevant files for testing

• compute_worker/compute_worker.py — Run._fetch_submission_state() + run_wrapper() short-circuit
• src/apps/api/views/submissions.py — worker_state endpoint, check_object_permissions guard, upload_submission_scores upsert
• src/apps/api/serializers/submissions.py — terminal status mutation guard
• src/apps/competitions/models.py — Submission.worker_attempt_count field
• Migrations: 0063_submission_worker_attempt_count.py

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCI tests are passing
• Ready to merge