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/ghost_content.py

@@ -164,6 +164,14 @@ class Post(Base):
         order_by="MenuItem.sort_order",
     )
 
+    page_config: Mapped[Optional["PageConfig"]] = relationship(
+        "PageConfig",
+        back_populates="post",
+        uselist=False,
+        cascade="all, delete-orphan",
+        passive_deletes=True,
+    )
+
 class Author(Base):
     __tablename__ = "authors"