fix(backend): harden write registrar boundaries

backend/internal/logic/tasks/handlers.go

@@ -93,15 +93,18 @@ func (h *Handler) update(c *gin.Context) {
 		httpx.Error(c, http.StatusBadRequest, "invalid_request", "请求参数无效")
 		return
 	}
+	nextProjectIdentity := ""
 	if strings.TrimSpace(input.NextProjectID) != "" {
-		if _, ok := parseIdentity(input.NextProjectID); !ok {
+		var valid bool
+		nextProjectIdentity, valid = parseIdentity(input.NextProjectID)
+		if !valid {
 			httpx.Error(c, http.StatusBadRequest, "invalid_identity", "nextProjectId 必须是 UUIDv7")
 			return
 		}
 	}
 	task, err := h.service.Update(userID, projectIdentity, taskIdentity, UpdateTaskInput{
 		Title: input.Title, Description: input.Description, Status: input.Status, Completed: input.Completed,
-		NextProjectIdentity: input.NextProjectID, Tag: input.Tag,
+		NextProjectIdentity: nextProjectIdentity, Tag: input.Tag,
 	})
 	if err != nil {
 		writeTaskError(c, err)