Avoid and detect breakages of the commit_pending mechanism

Celery can break because of corrupted database, which causes Weblate to stop committing to its repository.

One way to improve the situation seems to be to install python3-gdbm.

How to check the service is running correctly:

• Check that service celery-weblate is running.
• Check that latest heartbeat was <= 1min in /var/log/celery/weblate-celery.log.
• Check that last commit-pending was <= 1h in /var/log/celery/weblate-celery.log.
• https://docs.celeryproject.org/en/stable/userguide/monitoring.html