Add scheduled cleanup job that removes old completed and failed queue
jobs after 30 days. Jobs are kept for debugging purposes.
Includes migration to change the next_attempt_id FK constraint from NO
ACTION to SET NULL, allowing cleanup of retry chains without breaking
foreign key constraints.
One caveat is that cleanup is based on their creation time, *not* when
they got completed/failed. This means that if the job takes a long time
(as in, several days) to get scheduled, it might get cleared as soon as
it runs. This is fine for now, we may want to revisit this if we start
scheduling jobs far in the future
Quentin Gliech