fix(audit-g3): lifecycle batch — recovery sweeps, retention, timeouts

Plugs the FC long-running-entity discipline gaps the 2026-06-02 audit
flagged: every entity that can get stuck now has recovery + retention +
timeout, and the long-runners no longer collide with the FC-3i sweep.

Recovery sweeps (every 5 min):
- recover_stalled_backup_runs — flips BackupRun stuck in
  running/restoring past 7h (covers the 6.5h images-backup hard
  limit) to error. prune_backups docstring corrected — the FC-3i
  TaskRun sweep never touched BackupRun rows.
- recover_stalled_library_audit_runs — flips LibraryAuditRun stuck
  past 135 min (10-min buffer above scan_library_for_rule's 2h5m
  hard limit) to error. Previously a SIGKILL'd row blocked all
  future audits until manual DB surgery.
- recover_stalled_import_batches — finalizes ImportBatch rows
  stuck running >2h whose child tasks are all terminal (orphan case
  where the orchestrator crashed before the closing UPDATE). Uses
  the same EXISTS predicate /api/system/stats already had.

Retention (daily):
- prune_library_audit_runs — 30-day window. Audit rows carry
  matched_ids JSONB blobs that can hold tens of thousands of ids.
- prune_import_batches — 30-day window. Cascades to ImportTask via
  the model relationship.

time_limits on five long-runners that previously had none (the
audit's headline finding — every one of these collided with the
recover_stalled_task_runs 5-min default and could be marked
'error' mid-flight):
- scan_directory: 60m soft / 70m hard
- verify_integrity: 60m / 70m
- backfill_phash: 30m / 35m
- apply_allowlist_tags: 30m / 35m
- recompute_centroids: 30m / 35m

QUEUE_STUCK_THRESHOLD_MINUTES now covers maintenance (75) and scan
(75) — above the longest task on each — with per-task overrides
for the outliers (backup_images_task 420, restore_images_task 420,
scan_library_for_rule 130).

start_audit_run guard is now age-aware: a 'running' row older than
the audit hard limit doesn't block a new run (the sweep will catch
it within 5 min). Previously a SIGKILL'd row blocked forever.

/api/import/status now uses the same EXISTS predicate
/api/system/stats does, so the two endpoints no longer disagree on
the active-batch question.

DownloadEvent.started_at resets on pending→running so a freshly-
promoted event from a busy queue isn't measured against its
original enqueue time (was racing recover_stalled_download_events
on heavy-queue days).

backend/app/tasks/ml.py

@@ -212,7 +212,14 @@ def backfill(self) -> int:
     return enqueued
 
 
-@celery.task(name="backend.app.tasks.ml.apply_allowlist_tags", bind=True)
+@celery.task(
+    name="backend.app.tasks.ml.apply_allowlist_tags",
+    bind=True,
+    # Audit 2026-06-02 — the full-sweep mode (neither tag_id nor image_id)
+    # is O(images × allowlist) and legitimately runs >5 min on large
+    # libraries. Cap matches the maintenance queue's recovery threshold.
+    soft_time_limit=1800, time_limit=2100,
+)
 def apply_allowlist_tags(self, tag_id: int | None = None,
                          image_id: int | None = None) -> int:
     """Retroactively apply allowlisted tags.
@@ -341,7 +348,13 @@ def recompute_centroid(self, tag_id: int) -> bool:
     return asyncio.run(_run())
 
 
-@celery.task(name="backend.app.tasks.ml.recompute_centroids", bind=True)
+@celery.task(
+    name="backend.app.tasks.ml.recompute_centroids",
+    bind=True,
+    # Audit 2026-06-02 — drifted-centroid rebuild over potentially
+    # hundreds of tags.
+    soft_time_limit=1800, time_limit=2100,
+)
 def recompute_centroids(self) -> int:
     """Daily: find drifted centroids, enqueue recompute_centroid for each."""
     import asyncio