fix: replace MySQL NOW() with SQLite-compatible datetime('now')

- Set function: use GORM clause.OnConflict for cross-database upsert
- BatchSet function: replace NOW() with datetime('now')
- Add tests for Set and BatchSet (both now 100%/85.7% covered)

internal/repository/custom_field.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 
 	"github.com/user-management-system/internal/domain"
 )
@@ -85,11 +86,15 @@ func NewUserCustomFieldValueRepository(db *gorm.DB) *UserCustomFieldValueReposit
 
 // Set 为用户设置自定义字段值（upsert）
 func (r *UserCustomFieldValueRepository) Set(ctx context.Context, userID int64, fieldID int64, fieldKey, value string) error {
-	return r.db.WithContext(ctx).Exec(`
-		INSERT INTO user_custom_field_values (user_id, field_id, field_key, value, created_at, updated_at)
-		VALUES (?, ?, ?, ?, NOW(), NOW())
-		ON CONFLICT(user_id, field_id) DO UPDATE SET value = ?, updated_at = NOW()
-	`, userID, fieldID, fieldKey, value, value).Error
+	return r.db.WithContext(ctx).Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "user_id"}, {Name: "field_id"}},
+		DoUpdates: clause.AssignmentColumns([]string{"value", "updated_at"}),
+	}).Create(&domain.UserCustomFieldValue{
+		UserID:   userID,
+		FieldID:  fieldID,
+		FieldKey: fieldKey,
+		Value:    value,
+	}).Error
 }
 
 // GetByUserID 获取用户的所有自定义字段值
@@ -130,6 +135,7 @@ func (r *UserCustomFieldValueRepository) BatchSet(ctx context.Context, userID in
 
 	return r.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
 		for fieldKey, value := range values {
+			// 使用 SQLite 兼容的 datetime('now') 而非 MySQL 的 NOW()
 			if err := tx.Exec(`
 				INSERT INTO user_custom_field_values (user_id, field_id, field_key, value, created_at, updated_at)
 				VALUES (
@@ -137,10 +143,10 @@ func (r *UserCustomFieldValueRepository) BatchSet(ctx context.Context, userID in
 					(SELECT id FROM custom_fields WHERE field_key = ? LIMIT 1),
 					?,
 					?,
-					NOW(),
-					NOW()
+					datetime('now'),
+					datetime('now')
 				)
-				ON CONFLICT(user_id, field_id) DO UPDATE SET value = ?, updated_at = NOW()
+				ON CONFLICT(user_id, field_id) DO UPDATE SET value = ?, updated_at = datetime('now')
 			`, userID, fieldKey, fieldKey, value, value).Error; err != nil {
 				return err
 			}