feat(fc2a): add maintenance tasks (recovery + cleanup) with beat schedule

recover_interrupted_tasks runs every 5 minutes, finds ImportTask rows
stuck in 'processing' for >30 minutes (well above any legitimate import
duration), and re-queues them. cleanup_old_tasks runs daily and deletes
finished tasks older than 7 days so the task table stays an operational
view rather than an archive.

Both thresholds match ImageRepo's precedent. The 30-min stuck threshold
is documented inline so a future reader can adjust it intentionally
rather than mistaking it for a 'magic number'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/celery_app.py

@@ -44,6 +44,17 @@ def make_celery() -> Celery:
         task_acks_late=True,
         worker_prefetch_multiplier=1,
         broker_connection_retry_on_startup=True,
+        beat_schedule={
+            "recover-interrupted-tasks": {
+                "task": "backend.app.tasks.maintenance.recover_interrupted_tasks",
+                "schedule": 300.0,  # every 5 minutes
+            },
+            "cleanup-old-tasks": {
+                "task": "backend.app.tasks.maintenance.cleanup_old_tasks",
+                "schedule": 86400.0,  # daily
+            },
+        },
+        timezone="UTC",
     )
     return app