feat: per-page SumUp models and migration (Phase 3)

- Migration: add market_place_id to cart_items, page_config_id to orders
- Order model: add page_config_id FK and page_config relationship
- CartItem model: add market_place_id FK and market_place relationship

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

models/market.py

@@ -413,13 +413,23 @@ class CartItem(Base):
         nullable=False,
         server_default=func.now(),
     )
+    market_place_id: Mapped[int | None] = mapped_column(
+        ForeignKey("market_places.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
     deleted_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True),
         nullable=True,
     )
 
     # Relationships
-   
+
+    market_place: Mapped["MarketPlace | None"] = relationship(
+        "MarketPlace",
+        foreign_keys=[market_place_id],
+    )
     product: Mapped["Product"] = relationship(
         "Product",
         back_populates="cart_items",