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>

infrastructure/factory.py

@@ -11,7 +11,7 @@ from shared.config import init_config, config, pretty
 from shared.models import KV  # ensure shared models imported
 # Register all app model classes with SQLAlchemy so cross-domain
 # relationship() string references resolve correctly.
-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:
@@ -144,6 +144,11 @@ def create_base_app(
     # --- startup ---
     @app.before_serving
     async def _startup():
+        try:
+            from glue.setup import register_glue_handlers
+            register_glue_handlers()
+        except ImportError:
+            pass  # glue submodule not present in this build context
         await init_config()
         print(pretty())
         await _event_processor.start()