Add glue layer support: MenuNode templates, factory registration, migration

- Templates: item.post.X → item.X (MenuNode has label/slug/feature_image directly)
- factory.py: add glue.models to import loop + register_glue_handlers() at startup
- alembic env.py: add glue.models to import loop
- New migration: container_relations + menu_nodes tables with backfill from existing data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/env.py

@@ -19,7 +19,7 @@ from shared.db.base import Base
 
 # Import ALL models so Base.metadata sees every table
 import shared.models  # noqa: F401  User, KV, MagicLink, MenuItem, Ghost*
-for _mod in ("blog.models", "market.models", "cart.models", "events.models"):
+for _mod in ("blog.models", "market.models", "cart.models", "events.models", "glue.models"):
     try:
         __import__(_mod)
     except ImportError: