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/order.py

@@ -17,6 +17,12 @@ class Order(Base):
     user_id: Mapped[Optional[int]] = mapped_column(ForeignKey("users.id"), nullable=True)
     session_id: Mapped[Optional[str]] = mapped_column(String(64), index=True, nullable=True)
 
+    page_config_id: Mapped[Optional[int]] = mapped_column(
+        ForeignKey("page_configs.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+
     status: Mapped[str] = mapped_column(
         String(32),
         nullable=False,
@@ -68,6 +74,11 @@ class Order(Base):
         back_populates="order",
         lazy="selectin",
     )
+    page_config: Mapped[Optional["PageConfig"]] = relationship(
+        "PageConfig",
+        foreign_keys=[page_config_id],
+        lazy="selectin",
+    )
 
 
 class OrderItem(Base):