fix: save cart instance before iterating

Author: henri-hulski

cartridge/shop/models.py

@@ -607,6 +607,7 @@ def __iter__(self):
         ensuring the items are only retrieved once and cached.
         """
         if not hasattr(self, "_cached_items"):
+            self.save()
             self._cached_items = self.items.all()
         return iter(self._cached_items)