fix(backend): harden write registrar boundaries

backend/internal/logic/projects/cron_handlers.go

@@ -1,11 +1,13 @@
 package projects
 
 import (
+	"errors"
 	"net/http"
 	"strings"
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
 	"senlinai-agent/backend/internal/httpx"
 	"senlinai-agent/backend/internal/models"
 )
@@ -62,12 +64,23 @@ func (h *CronHandler) create(c *gin.Context) {
 		Title: input.Title, Schedule: input.Schedule, Enabled: input.Enabled, NextRunAt: nextRunAt,
 	})
 	if err != nil {
-		httpx.Error(c, http.StatusBadRequest, "invalid_request", "计划任务参数无效")
+		writeCronError(c, err)
 		return
 	}
 	c.JSON(http.StatusCreated, cronPlanDTO(*plan))
 }
 
+func writeCronError(c *gin.Context, err error) {
+	switch {
+	case errors.Is(err, ErrCronTitleRequired), errors.Is(err, ErrCronScheduleRequired):
+		httpx.Error(c, http.StatusBadRequest, "invalid_request", "计划任务参数无效")
+	case errors.Is(err, gorm.ErrRecordNotFound):
+		httpx.Error(c, http.StatusNotFound, "not_found", "项目不存在")
+	default:
+		httpx.Error(c, http.StatusInternalServerError, "internal_error", "计划任务创建失败")
+	}
+}
+
 func cronPlanDTO(plan models.SenlinAgentCronPlan) CronPlanDTO {
 	return CronPlanDTO{
 		ID: plan.Identity, ProjectID: plan.ProjectIdentity, Title: plan.Title, Schedule: plan.Schedule,