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>

bp/orders/routes.py

@@ -5,8 +5,8 @@ from sqlalchemy import select, func, or_, cast, String, exists
 from sqlalchemy.orm import selectinload
 
 
-from market.models.market import Product
-from models.order import Order, OrderItem
+from shared.models.market import Product
+from shared.models.order import Order, OrderItem
 from shared.browser.app.payments.sumup import create_checkout as sumup_create_checkout
 from shared.config import config