feat(fc2b): add tag_and_embed + backfill Celery tasks

tag_and_embed: Camie + SigLIP on one image (video → 10-frame sample,
max-pool tags, mean-pool embeddings), stores predictions/embedding with
model versions, then enqueues per-image allowlist apply. backfill:
keyset-paginated discovery of images missing predictions/embeddings for
the current model versions (restart-safe). apply_allowlist_tags stub
included so .delay() resolves between commits (filled in Task 9).

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

backend/app/celery_app.py

@@ -28,6 +28,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.import_file",
             "backend.app.tasks.thumbnail",
             "backend.app.tasks.maintenance",
+            "backend.app.tasks.ml",
         ],
     )
     app.conf.update(