fix: 添加分类名称唯一约束，防止重复名称

backend/internal/api/handlers/product.go

@@ -34,6 +34,12 @@ func (h *CategoryHandler) Create(c *gin.Context) {
 		return
 	}
 
+	var existing models.Category
+	if err := utils.DB.Where("name = ?", req.Name).First(&existing).Error; err == nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "分类名称已存在"})
+		return
+	}
+
 	category := models.Category{
 		Name:        req.Name,
 		Description: req.Description,
@@ -69,6 +75,14 @@ func (h *CategoryHandler) Update(c *gin.Context) {
 		return
 	}
 
+	if req.Name != nil && *req.Name != category.Name {
+		var existing models.Category
+		if err := utils.DB.Where("name = ? AND id != ?", *req.Name, id).First(&existing).Error; err == nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "分类名称已存在"})
+			return
+		}
+	}
+
 	updates := make(map[string]interface{})
 	if req.Name != nil {
 		updates["name"] = *req.Name