fix: resolve async SQLAlchemy lazy loading issue in cart service

Use get_cart() with selectinload instead of refresh() to avoid
greenlet_spawn errors when accessing cart.items relationship.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/cart_service.py

@@ -21,7 +21,13 @@ class CartService:
         cart = Cart()
         self.db.add(cart)
         await self.db.commit()
-        await self.db.refresh(cart)
+        # Re-fetch with items loaded to avoid lazy loading issues
+        result = await self.db.execute(
+            select(Cart)
+            .where(Cart.id == cart.id)
+            .options(selectinload(Cart.items))
+        )
+        cart = result.scalar_one()
         return self._cart_to_response(cart)
 
     async def get_cart(self, cart_id: UUID) -> CartResponse | None:
@@ -59,8 +65,7 @@ class CartService:
             ):
                 existing.quantity += item.quantity
                 await self.db.commit()
-                await self.db.refresh(cart)
-                return self._cart_to_response(cart)
+                return await self.get_cart(cart_id)
 
         # Add new item
         cart_item = CartItem(
@@ -73,8 +78,7 @@ class CartService:
         )
         self.db.add(cart_item)
         await self.db.commit()
-        await self.db.refresh(cart)
-        return self._cart_to_response(cart)
+        return await self.get_cart(cart_id)
 
     async def update_item(
         self,