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/tasks/maintenance.py

@@ -473,6 +473,10 @@ def prune_task_runs() -> dict:
       (recover_stalled_task_runs) is the mechanism that flips them to
       terminal state; prune doesn't touch in-flight state.
     - 'retry' rows: treated as failures (>7d).
+    - The NEWEST row of each task is never deleted, whatever its age: it is
+      what the beat scheduler reads to know when a job last ran (#4408).
+      Without it a weekly job's last run would be pruned after a day, and beat
+      would think it had never run and fire it on every restart.
 
     Returns dict of how many rows were deleted in each bucket.
     """
@@ -480,16 +484,19 @@ def prune_task_runs() -> dict:
     now = datetime.now(UTC)
     ok_cutoff = now - timedelta(seconds=TASK_RUN_KEEP_OK_SECONDS)
     fail_cutoff = now - timedelta(seconds=TASK_RUN_KEEP_FAILURE_SECONDS)
+    newest = select(func.max(TaskRun.id)).group_by(TaskRun.task_name)
     with SessionLocal() as session:
         ok_deleted = session.execute(
             delete(TaskRun)
             .where(TaskRun.status == "ok")
             .where(TaskRun.finished_at < ok_cutoff)
+            .where(TaskRun.id.not_in(newest))
         ).rowcount or 0
         fail_deleted = session.execute(
             delete(TaskRun)
             .where(TaskRun.status.in_(["error", "timeout", "retry"]))
             .where(TaskRun.finished_at < fail_cutoff)
+            .where(TaskRun.id.not_in(newest))
         ).rowcount or 0
         session.commit()
         return {"ok_deleted": ok_deleted, "failures_deleted": fail_deleted}