fix(backend): harden final MVP invariants

backend/internal/logic/projects/object_crud.go

@@ -3,8 +3,10 @@ package projects
 import (
 	"errors"
 	"strings"
+	"time"
 
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 	"senlinai-agent/backend/internal/models"
 )
 
@@ -28,7 +30,7 @@ func (s *Service) CreateCronPlan(ownerID uint, projectID uint, input CreateCronP
 	}
 	plan := &models.SenlinAgentCronPlan{
 		ProjectID: projectID, CreatedBy: ownerID, Title: title, Schedule: schedule,
-		Enabled: input.Enabled, NextRunAt: input.NextRunAt, LastResult: "Not run yet",
+		Enabled: input.Enabled, NextRunAt: input.NextRunAt,
 	}
 	return plan, models.DBService.Create(plan).Error
 }
@@ -45,16 +47,40 @@ func (s *Service) CreateTag(projectID uint, name string) (*models.SenlinAgentTag
 	if name == "" {
 		return nil, ErrTagNameRequired
 	}
-	var existing models.SenlinAgentTag
-	err := models.DBService.Where("project_id = ? AND name = ?", projectID, name).First(&existing).Error
-	if err == nil {
-		return &existing, nil
+	var lastErr error
+	for attempt := 0; attempt < 5; attempt++ {
+		candidate := models.SenlinAgentTag{ProjectID: projectID, Name: name}
+		lastErr = models.DBService.Clauses(clause.OnConflict{
+			Columns:   []clause.Column{{Name: "project_id"}, {Name: "name"}},
+			DoNothing: true,
+		}).Create(&candidate).Error
+		if lastErr == nil {
+			var stored models.SenlinAgentTag
+			lastErr = models.DBService.Where("project_id = ? AND name = ?", projectID, name).First(&stored).Error
+			if lastErr == nil {
+				return &stored, nil
+			}
+		}
+		if !retryableTagWrite(lastErr) {
+			return nil, lastErr
+		}
+		time.Sleep(time.Duration(attempt+1) * time.Millisecond)
 	}
-	if !errors.Is(err, gorm.ErrRecordNotFound) {
-		return nil, err
+	return nil, lastErr
+}
+
+func retryableTagWrite(err error) bool {
+	if errors.Is(err, gorm.ErrDuplicatedKey) || errors.Is(err, gorm.ErrRecordNotFound) || strings.Contains(strings.ToLower(err.Error()), "database is locked") {
+		return true
 	}
-	tag := &models.SenlinAgentTag{ProjectID: projectID, Name: name}
-	return tag, models.DBService.Create(tag).Error
+	var sqlState interface{ SQLState() string }
+	if errors.As(err, &sqlState) {
+		switch sqlState.SQLState() {
+		case "40001", "40P01", "55P03":
+			return true
+		}
+	}
+	return false
 }
 
 func (s *Service) ListProjectTags(ownerID uint, projectID uint) ([]models.SenlinAgentTag, error) {