Move container_relations to cart service for cross-service ownership

container_relations is a generic parent/child graph used by blog
(menu_nodes), market (marketplaces), and events (calendars). Move it
to cart as shared infrastructure. All services now call cart actions
(attach-child/detach-child) instead of querying the table directly.

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

cart/alembic/env.py

@@ -5,10 +5,11 @@ MODELS = [
     "shared.models.market",       # CartItem lives here
     "shared.models.order",
     "shared.models.page_config",
+    "shared.models.container_relation",
 ]
 
 TABLES = frozenset({
-    "cart_items", "orders", "order_items", "page_configs",
+    "cart_items", "orders", "order_items", "page_configs", "container_relations",
 })
 
 run_alembic(context.config, MODELS, TABLES)