feat(fc2b): add tag rename (PATCH /api/tags/<id>) + /api/ml admin

Rename returns 409 on collision (frontend shows FC-2c merge hint), 400
on empty. ml_admin: GET/PATCH settings (thresholds + min_reference;
model versions read-only here), POST backfill / recompute-centroids
returning 202 + celery task id. Tests integration-marked, eager Celery.

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

backend/app/api/__init__.py

@@ -18,6 +18,7 @@ def all_blueprints() -> list[Blueprint]:
     from .allowlist import allowlist_bp
     from .gallery import gallery_bp
     from .import_admin import import_admin_bp
+    from .ml_admin import ml_admin_bp
     from .settings import settings_bp
     from .suggestions import suggestions_bp
     from .tags import tags_bp
@@ -30,4 +31,5 @@ def all_blueprints() -> list[Blueprint]:
         suggestions_bp,
         allowlist_bp,
         aliases_bp,
+        ml_admin_bp,
     ]