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>

bp/coop_api.py

@@ -2,50 +2,18 @@
 Internal JSON API for the coop app.
 
 These endpoints are called by other apps (market, cart) over HTTP
-to fetch Ghost CMS content and menu items without importing blog services.
+to fetch Ghost CMS content without importing blog services.
 """
 from __future__ import annotations
 
 from quart import Blueprint, g, jsonify
-from sqlalchemy import select
-from sqlalchemy.orm import selectinload
 
-from shared.models.menu_item import MenuItem
 from shared.browser.app.csrf import csrf_exempt
 
 
 def register() -> Blueprint:
     bp = Blueprint("coop_api", __name__, url_prefix="/internal")
 
-    @bp.get("/menu-items")
-    @csrf_exempt
-    async def menu_items():
-        """
-        Return all active menu items as lightweight JSON.
-        Called by market and cart apps to render the nav.
-        """
-        result = await g.s.execute(
-            select(MenuItem)
-            .where(MenuItem.deleted_at.is_(None))
-            .options(selectinload(MenuItem.post))
-            .order_by(MenuItem.sort_order.asc(), MenuItem.id.asc())
-        )
-        items = result.scalars().all()
-
-        return jsonify(
-            [
-                {
-                    "id": mi.id,
-                    "post": {
-                        "title": mi.post.title if mi.post else None,
-                        "slug": mi.post.slug if mi.post else None,
-                        "feature_image": mi.post.feature_image if mi.post else None,
-                    },
-                }
-                for mi in items
-            ]
-        )
-
     @bp.get("/post/<slug>")
     @csrf_exempt
     async def post_by_slug(slug: str):