fix: beat takes each job's last run from task_run, so a redeploy no longer resets the schedule (4408)

Beat's default scheduler kept its memory in a shelve file nothing persists,
and a scheduler that remembers nothing waits a full interval before any job.
Since the one-container image, every redeploy restarts beat, and no daily or
weekly job had run since 2026-09-21 (cleanup, backup and download-event
pruning, membership sync, thumbnail backfill, integrity check, vacuum).

TaskRunScheduler seeds each entry's last_run_at at startup from task_run's
newest start for that task: an overdue job runs at once, one not yet due
waits the remainder, and one never recorded is due now. prune_task_runs now
keeps each task's newest row however old, or a weekly job would look
never-run a day after it ran and fire on every restart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/celery_app.py

@@ -353,6 +353,9 @@ def make_celery() -> Celery:
             },
         },
         timezone="UTC",
+        # Beat's memory of when each job last ran comes from task_run, not a
+        # shelve file nothing persists — see beat_scheduler (#4408).
+        beat_scheduler="backend.app.beat_scheduler:TaskRunScheduler",
     )
     # FC-3i: register task_run signal handlers (side-effect import).
     from . import celery_signals  # noqa: F401