chore(beat): remove non-existent refresh_compute_worker_health task

[AybH26]

Closes #2410

Context

CELERY_BEAT_SCHEDULE in src/settings/base.py contained an entry that published competitions.tasks.refresh_compute_worker_health every 60s. That task does not exist anywhere in the codebase, so Beat was publishing a name the worker could not resolve.

Effect

Repeated NotRegistered / KeyError errors on the site_worker prefork consumer every minute. Log noise, and under certain conditions the consumer channel can be left in a broken state, blocking other tasks queued on site-worker.

Change

• Remove the dead entry from CELERY_BEAT_SCHEDULE in src/settings/base.py.

How to verify

1. Start the stack: docker compose up site_worker.
2. Observe site_worker logs for one minute.
3. Before: a Received unregistered task of type 'competitions.tasks.refresh_compute_worker_health' (or equivalent KeyError) appears every ~60s.
4. After: no such error appears.

Risk

Minimal. The removed task was never executed (it does not exist), so no runtime behaviour depended on it.

[Didayolo]

Thank you for the PR and for following our guidelines. We'll review it as soon as possible.