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/schemas/product.go

@@ -30,6 +30,7 @@ type CreateProductRequest struct {
 	Name          string                 `json:"name" binding:"required"`
 	Description   string                 `json:"description"`
 	Price         float64                `json:"price" binding:"gte=0"`
+	Weight        float64                `json:"weight"`
 	MinPurchase   int                    `json:"min_purchase"`
 	MaxPurchase   *int                   `json:"max_purchase"`
 	MinWeight     *float64               `json:"min_weight"`
@@ -49,6 +50,7 @@ type UpdateProductRequest struct {
 	Name          *string                `json:"name"`
 	Description   *string                `json:"description"`
 	Price         *float64               `json:"price"`
+	Weight        *float64               `json:"weight"`
 	MinPurchase   *int                   `json:"min_purchase"`
 	MaxPurchase   *int                   `json:"max_purchase"`
 	MinWeight     *float64               `json:"min_weight"`