chore: retire ml-backfill-daily beat + the spent purge-legacy action (operator-approved)

- ml-backfill-daily: the CPU tag_and_embed backfill raced the GPU agent's
  daily embed backfill for the same NULL-embedding images at ~100x the cost
  (B1 audit verdict, milestone #124). The backfill TASK stays — the manual
  /api/ml/backfill button remains the deliberate CPU fallback pending B3.
- purge-legacy: one-time IR-migration cleanup, dry-run verified 0 targets on
  the live library before removal (A2 audit, milestone #123). Fully retired
  per rule 22: tile, store action, route, service fn, tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/celery_app.py

@@ -97,10 +97,6 @@ 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,
-            },
             "train-heads-nightly": {
                 "task": "backend.app.tasks.ml.scheduled_train_heads",
                 "schedule": 86400.0,  # passive cadence; manual retrain stays available