Decouple cross-app models: move canonical definitions to shared/models/

- Move 6 model files to shared/models/ (ghost_content, order, page_config,
  market, market_place, calendars) so apps import from shared, not each other
- Fix auth templates: replace url_for('auth.*') with coop_url() for
  cross-app compatibility
- Fix TYPE_CHECKING import in sumup.py to use shared.models.order
- Delete dead infrastructure/cart_loader.py (inverted dependency)
- Update models/__init__.py to export all new models

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

browser/app/payments/sumup.py

@@ -9,7 +9,7 @@ from quart import current_app
 from shared.config import config
 
 if TYPE_CHECKING:
-    from cart.models.order import Order
+    from shared.models.order import Order
 
 SUMUP_BASE_URL = "https://api.sumup.com/v0.1"