fix(fc2b): add ML beat schedule entries (missed in Task 9 commit)

The celery_app.py beat-schedule edit failed with a stale-read error in
the Task 9 commit, so the ML daily jobs weren't registered. Adds
ml-backfill-daily, recompute-centroids-daily, apply-allowlist-sweep-daily.

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

backend/app/celery_app.py

@@ -54,6 +54,18 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.cleanup_old_tasks",
                 "schedule": 86400.0,  # daily
             },
+            "ml-backfill-daily": {
+                "task": "backend.app.tasks.ml.backfill",
+                "schedule": 86400.0,
+            },
+            "recompute-centroids-daily": {
+                "task": "backend.app.tasks.ml.recompute_centroids",
+                "schedule": 86400.0,
+            },
+            "apply-allowlist-sweep-daily": {
+                "task": "backend.app.tasks.ml.apply_allowlist_tags",
+                "schedule": 86400.0,
+            },
         },
         timezone="UTC",
     )