fix: preserve successful storage target updates

server/internal/repository/backup_record_repository.go

@@ -115,7 +115,9 @@ func (r *GormBackupRecordRepository) Create(ctx context.Context, item *model.Bac
 }
 
 func (r *GormBackupRecordRepository) Update(ctx context.Context, item *model.BackupRecord) error {
-	return r.db.WithContext(ctx).Save(item).Error
+	// Task 与 StorageTarget 是查询时预加载的只读关联。更新记录字段时忽略它们，
+	// 避免已加载的旧关联把刚修改的外键（例如首个成功上传目标）覆盖回去。
+	return r.db.WithContext(ctx).Omit("Task", "StorageTarget").Save(item).Error
 }
 
 func (r *GormBackupRecordRepository) Delete(ctx context.Context, id uint) error {

server/internal/repository/backup_record_repository_test.go

@@ -88,6 +88,21 @@ func TestBackupRecordRepositoryQueries(t *testing.T) {
 	if recent[0].StorageTarget.Name != "local" {
 		t.Fatalf("expected recent storage target to be preloaded, got %#v", recent[0].StorageTarget)
 	}
+	secondTarget := &model.StorageTarget{Name: "secondary", Type: "local_disk", Enabled: true, ConfigCiphertext: "{}", ConfigVersion: 1, LastTestStatus: "unknown"}
+	if err := repo.db.Create(secondTarget).Error; err != nil {
+		t.Fatalf("seed second storage target error: %v", err)
+	}
+	stored.StorageTargetID = secondTarget.ID
+	if err := repo.Update(ctx, stored); err != nil {
+		t.Fatalf("Update storage target ID returned error: %v", err)
+	}
+	updated, err := repo.FindByID(ctx, record.ID)
+	if err != nil {
+		t.Fatalf("FindByID after update returned error: %v", err)
+	}
+	if updated == nil || updated.StorageTargetID != secondTarget.ID || updated.StorageTarget.Name != "secondary" {
+		t.Fatalf("expected updated storage target to remain %d, got %#v", secondTarget.ID, updated)
+	}
 	total, err := repo.Count(ctx)
 	if err != nil {
 		t.Fatalf("Count returned error: %v", err)