fix: 修复商品克重保存问题

- schema添加Weight字段
- Create和Update处理中添加Weight字段支持

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/handlers/product.go

@@ -236,6 +236,7 @@ func (h *ProductHandler) Create(c *gin.Context) {
 		Name:          req.Name,
 		Description:   req.Description,
 		Price:         req.Price,
+		Weight:        req.Weight,
 		MinPurchase:   req.MinPurchase,
 		MaxPurchase:   req.MaxPurchase,
 		MinWeight:     req.MinWeight,
@@ -299,6 +300,9 @@ func (h *ProductHandler) Update(c *gin.Context) {
 	if req.Price != nil {
 		updates["price"] = *req.Price
 	}
+	if req.Weight != nil {
+		updates["weight"] = *req.Weight
+	}
 	if req.MinPurchase != nil {
 		updates["min_purchase"] = *req.MinPurchase
 	}