feat(maintenance): daily janitor for orphaned .part/.partial staging files

Downloads/imports stage into <name>.part / <name>.partial then os.replace() into
place, so a kill mid-write leaves a discardable temp — never a corrupt final.
cleanup_orphaned_temp_files sweeps ones left behind under the images root, only
older than 6h so an in-flight download's staging file is never removed. Daily beat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/celery_app.py

@@ -115,6 +115,11 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.cleanup_old_tasks",
                 "schedule": 86400.0,  # daily
             },
+            "cleanup-orphaned-temp-files": {
+                "task": "backend.app.tasks.maintenance.cleanup_orphaned_temp_files",
+                "schedule": 86400.0,  # daily — sweep .part/.partial left by a
+                # download/import killed mid-write (graceful-shutdown fallout)
+            },
             "train-heads-nightly": {
                 "task": "backend.app.tasks.ml.scheduled_train_heads",
                 "schedule": 86400.0,  # passive cadence; manual retrain stays available