fix: preserve legacy time casts throughout index migrations

internal/data/business/registration_index.go

@@ -44,11 +44,20 @@ func migrateRegistrationIndex(db *gorm.DB) error {
 }
 
 func addRegistrationIndex(db *gorm.DB) error {
+	return withLegacyRegistrationCast(db, func(conn *gorm.DB) error {
+		return conn.Exec("ALTER TABLE tab_user ADD INDEX idx_sw_user_registration(source_db,puid,sw_register_time_unix,id), ALGORITHM=INPLACE, LOCK=NONE").Error
+	})
+}
+
+func withLegacyRegistrationCast(db *gorm.DB, migrate func(*gorm.DB) error) error {
 	// 旧 SELECT CAST 会以警告方式接受尾部空格等历史值。建索引属于 DDL，
 	// 严格模式会把同一警告升级为错误；仅在固定连接的索引构建期间保持旧转换行为。
 	return db.Connection(func(conn *gorm.DB) (result error) {
+		// Keep the bound connection's base handle free of statement errors so
+		// restoration still executes after a failed ALTER TABLE.
+		fresh := func() *gorm.DB { return conn.Session(&gorm.Session{NewDB: true}) }
 		var previous string
-		if err := conn.Raw("SELECT @@session.sql_mode").Scan(&previous).Error; err != nil {
+		if err := fresh().Raw("SELECT @@session.sql_mode").Scan(&previous).Error; err != nil {
 			return err
 		}
 		modes := []string{}
@@ -57,14 +66,14 @@ func addRegistrationIndex(db *gorm.DB) error {
 				modes = append(modes, mode)
 			}
 		}
-		if err := conn.Exec("SET SESSION sql_mode=?", strings.Join(modes, ",")).Error; err != nil {
+		if err := fresh().Exec("SET SESSION sql_mode=?", strings.Join(modes, ",")).Error; err != nil {
 			return err
 		}
 		defer func() {
-			if err := conn.Exec("SET SESSION sql_mode=?", previous).Error; err != nil {
+			if err := fresh().Exec("SET SESSION sql_mode=?", previous).Error; err != nil {
 				result = errors.Join(result, err)
 			}
 		}()
-		return conn.Exec("ALTER TABLE tab_user ADD INDEX idx_sw_user_registration(source_db,puid,sw_register_time_unix,id), ALGORITHM=INPLACE, LOCK=NONE").Error
+		return migrate(fresh())
 	})
 }