fix(order): carry item weight through the contract-to-model transformer

CreateOrderContractToModel and AddToOrderContractToModel copied every order
item field except Weight, so a weight sent by the client never reached the
processor and every weight-based line failed with "product ... is sold by
weight and requires a weight".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/transformer/order_transformer.go

@@ -17,6 +17,7 @@ func CreateOrderContractToModel(req *contract.CreateOrderRequest) *models.Create
 			ProductID:        item.ProductID,
 			ProductVariantID: item.ProductVariantID,
 			Quantity:         item.Quantity,
+			Weight:           item.Weight,
 			UnitPrice:        item.UnitPrice, // Now optional
 			Modifiers:        item.Modifiers,
 			Notes:            item.Notes,
@@ -58,6 +59,7 @@ func AddToOrderContractToModel(req *contract.AddToOrderRequest) *models.AddToOrd
 			ProductID:        item.ProductID,
 			ProductVariantID: item.ProductVariantID,
 			Quantity:         item.Quantity,
+			Weight:           item.Weight,
 			UnitPrice:        item.UnitPrice, // Now optional
 			Modifiers:        item.Modifiers,
 			Notes:            item.Notes,