fix: unify handler response format in custom_field and role handlers

- custom_field_handler.go: Fix all error responses to use {code, message}
- role_handler.go: Fix all error responses to use {code, message}

Standardize all JSON responses to {code: 0, message: "success", data: ...} for success
and {code: XXX, message: "..."} for errors.

internal/api/handler/custom_field_handler.go

@@ -23,7 +23,7 @@ func NewCustomFieldHandler(customFieldService *service.CustomFieldService) *Cust
 func (h *CustomFieldHandler) CreateField(c *gin.Context) {
 	var req service.CreateFieldRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": err.Error()})
 		return
 	}
 
@@ -44,13 +44,13 @@ func (h *CustomFieldHandler) CreateField(c *gin.Context) {
 func (h *CustomFieldHandler) UpdateField(c *gin.Context) {
 	id, err := strconv.ParseInt(c.Param("id"), 10, 64)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid field id"})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "invalid field id"})
 		return
 	}
 
 	var req service.UpdateFieldRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": err.Error()})
 		return
 	}
 
@@ -71,7 +71,7 @@ func (h *CustomFieldHandler) UpdateField(c *gin.Context) {
 func (h *CustomFieldHandler) DeleteField(c *gin.Context) {
 	id, err := strconv.ParseInt(c.Param("id"), 10, 64)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid field id"})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "invalid field id"})
 		return
 	}
 
@@ -90,7 +90,7 @@ func (h *CustomFieldHandler) DeleteField(c *gin.Context) {
 func (h *CustomFieldHandler) GetField(c *gin.Context) {
 	id, err := strconv.ParseInt(c.Param("id"), 10, 64)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "invalid field id"})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": "invalid field id"})
 		return
 	}
 
@@ -126,7 +126,7 @@ func (h *CustomFieldHandler) ListFields(c *gin.Context) {
 func (h *CustomFieldHandler) SetUserFieldValues(c *gin.Context) {
 	userID, ok := getUserIDFromContext(c)
 	if !ok {
-		c.JSON(http.StatusUnauthorized, gin.H{"error": "unauthorized"})
+		c.JSON(http.StatusUnauthorized, gin.H{"code": 401, "message": "unauthorized"})
 		return
 	}
 
@@ -135,7 +135,7 @@ func (h *CustomFieldHandler) SetUserFieldValues(c *gin.Context) {
 	}
 
 	if err := c.ShouldBindJSON(&req); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		c.JSON(http.StatusBadRequest, gin.H{"code": 400, "message": err.Error()})
 		return
 	}
 
@@ -154,7 +154,7 @@ func (h *CustomFieldHandler) SetUserFieldValues(c *gin.Context) {
 func (h *CustomFieldHandler) GetUserFieldValues(c *gin.Context) {
 	userID, ok := getUserIDFromContext(c)
 	if !ok {
-		c.JSON(http.StatusUnauthorized, gin.H{"error": "unauthorized"})
+		c.JSON(http.StatusUnauthorized, gin.H{"code": 401, "message": "unauthorized"})
 		return
 	}