fix(backend): harden write registrar boundaries

backend/internal/logic/projects/object_crud.go

@@ -8,17 +8,23 @@ import (
 	"senlinai-agent/backend/internal/models"
 )
 
+var (
+	ErrTagNameRequired      = errors.New("tag name is required")
+	ErrCronTitleRequired    = errors.New("cron plan title is required")
+	ErrCronScheduleRequired = errors.New("cron schedule is required")
+)
+
 func (s *Service) CreateCronPlan(ownerID uint, projectID uint, input CreateCronPlanInput) (*models.SenlinAgentCronPlan, error) {
 	if err := ensureProjectOwner(ownerID, projectID); err != nil {
 		return nil, err
 	}
 	title := strings.TrimSpace(input.Title)
 	if title == "" {
-		return nil, errors.New("cron plan title is required")
+		return nil, ErrCronTitleRequired
 	}
 	schedule := strings.TrimSpace(input.Schedule)
 	if schedule == "" {
-		return nil, errors.New("cron schedule is required")
+		return nil, ErrCronScheduleRequired
 	}
 	plan := &models.SenlinAgentCronPlan{
 		ProjectID: projectID, CreatedBy: ownerID, Title: title, Schedule: schedule,
@@ -37,7 +43,7 @@ func (s *Service) CreateProjectTag(ownerID uint, projectID uint, name string) (*
 func (s *Service) CreateTag(projectID uint, name string) (*models.SenlinAgentTag, error) {
 	name = strings.TrimSpace(name)
 	if name == "" {
-		return nil, errors.New("tag name is required")
+		return nil, ErrTagNameRequired
 	}
 	var existing models.SenlinAgentTag
 	err := models.DBService.Where("project_id = ? AND name = ?", projectID, name).First(&existing).Error