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/bp/actions/routes.py

@@ -109,4 +109,47 @@ def register() -> Blueprint:
 
     _handlers["update-page-config"] = _update_page_config
 
+    # --- attach-child ---
+    async def _attach_child():
+        """Create or revive a ContainerRelation."""
+        from shared.services.relationships import attach_child
+
+        data = await request.get_json(force=True)
+        rel = await attach_child(
+            g.s,
+            parent_type=data["parent_type"],
+            parent_id=data["parent_id"],
+            child_type=data["child_type"],
+            child_id=data["child_id"],
+            label=data.get("label"),
+            sort_order=data.get("sort_order"),
+        )
+        return {
+            "id": rel.id,
+            "parent_type": rel.parent_type,
+            "parent_id": rel.parent_id,
+            "child_type": rel.child_type,
+            "child_id": rel.child_id,
+            "sort_order": rel.sort_order,
+        }
+
+    _handlers["attach-child"] = _attach_child
+
+    # --- detach-child ---
+    async def _detach_child():
+        """Soft-delete a ContainerRelation."""
+        from shared.services.relationships import detach_child
+
+        data = await request.get_json(force=True)
+        deleted = await detach_child(
+            g.s,
+            parent_type=data["parent_type"],
+            parent_id=data["parent_id"],
+            child_type=data["child_type"],
+            child_id=data["child_id"],
+        )
+        return {"deleted": deleted}
+
+    _handlers["detach-child"] = _detach_child
+
     return bp