fix: unify handler response format in user_handler.go

- List/Get/Update/Delete users: standardize to {code, message, data} format
- UpdateUserStatus: standardize to {code, message} format
- handleError: standardize to {code, message} format (was {error: ...})
- All inline bad request errors now use {code: 400, message: ...} consistently

internal/api/handler/auth_handler.go

@@ -417,14 +417,13 @@ func handleError(c *gin.Context, err error) {
 	// 优先尝试 ApplicationError（内置 HTTP 状态码）
 	var appErr *apierrors.ApplicationError
 	if errors.As(err, &appErr) {
-		c.JSON(int(appErr.Code), gin.H{"error": appErr.Message})
+		c.JSON(int(appErr.Code), gin.H{"code": appErr.Code, "message": appErr.Message})
 		return
 	}
 
 	// 对普通 errors.New 按关键词推断语义，但只返回通用错误信息给客户端
-	msg := err.Error()
-	code := classifyErrorMessage(msg)
-	c.JSON(code, gin.H{"error": "服务器内部错误"})
+	httpCode := classifyErrorMessage(err.Error())
+	c.JSON(httpCode, gin.H{"code": httpCode, "message": "服务器内部错误"})
 }
 
 // classifyErrorMessage 通过错误信息关键词推断 HTTP 状态码，避免业务错误被 500 吞掉