feat: add PageConfig model and remove duplicate CartItem

Introduce page_configs table for per-page feature flags (calendar, market)
and future SumUp credentials. Add page_config relationship to Post model.
Remove duplicate CartItem definition from cart_item.py (canonical stays in market.py).

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

models/__init__.py

@@ -13,6 +13,7 @@ from .ghost_membership_entities import (
 )
 
 from .calendars import Calendar, CalendarEntry, Ticket
+from .page_config import PageConfig