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>

models/ghost_content.py

@@ -141,14 +141,6 @@ class Post(Base):
         passive_deletes=True,
     )
 
-    menu_items: Mapped[List["MenuItem"]] = relationship(
-        "MenuItem",
-        back_populates="post",
-        cascade="all, delete-orphan",
-        passive_deletes=True,
-        order_by="MenuItem.sort_order",
-    )
-
 class Author(Base):
     __tablename__ = "authors"