Decouple cart: use shared.models for all cross-app imports

- Replace all imports from blog.models, market.models, events.models
  and bare models.* with shared.models equivalents
- Convert cart/models/order.py and page_config.py to re-export stubs
- Update shared + glue submodule pointers

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

app.py

@@ -81,8 +81,8 @@ async def cart_context() -> dict:
 
 
 def create_app() -> "Quart":
-    from blog.models.ghost_content import Post
-    from models.page_config import PageConfig
+    from shared.models.ghost_content import Post
+    from shared.models.page_config import PageConfig
 
     app = create_base_app(
         "cart",