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/data/routes.py

@@ -171,6 +171,32 @@ def register() -> Blueprint:
 
     _handlers["page-configs-batch"] = _page_configs_batch
 
+    # --- get-children ---
+    async def _get_children():
+        """Return ContainerRelation children for a parent."""
+        from shared.services.relationships import get_children
+
+        parent_type = request.args.get("parent_type", "")
+        parent_id = request.args.get("parent_id", type=int)
+        child_type = request.args.get("child_type")
+        if not parent_type or parent_id is None:
+            return []
+        rels = await get_children(g.s, parent_type, parent_id, child_type)
+        return [
+            {
+                "id": r.id,
+                "parent_type": r.parent_type,
+                "parent_id": r.parent_id,
+                "child_type": r.child_type,
+                "child_id": r.child_id,
+                "sort_order": r.sort_order,
+                "label": r.label,
+            }
+            for r in rels
+        ]
+
+    _handlers["get-children"] = _get_children
+
     return bp