Add glue layer: MenuNode replaces MenuItem, remove /internal/menu-items API

- Context processor: get_navigation_tree() replaces get_all_menu_items()
- Menu admin service: MenuItem → MenuNode (container_type/container_id pattern)
- Remove /internal/menu-items endpoint (other apps query menu_nodes directly)
- Remove menu_items relationship from Post model
- Templates: item.post.X → item.X
- Add glue submodule

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

.gitmodules

@@ -2,3 +2,6 @@
 	path = shared
 	url = https://git.rose-ash.com/coop/shared.git
 	branch = decoupling
+[submodule "glue"]
+	path = glue
+	url = /root/rose-ash/glue