refactor(mysql): 适配PXC严格模式，替换全局锁方案

重构MySQL迁移逻辑，新增acquireMigrationLock适配Percona XtraDB Cluster的pxc_strict_mode限制，将原有的GET_LOCK迁移锁替换为更兼容的方案，同时为各迁移流程添加快速检查路径减少锁竞争

server/db/mysql/snowflake_primary_keys.go

@@ -58,15 +58,16 @@ func ensureSnowflakePrimaryKeys(db *sqlx.DB) error {
 	}
 	defer conn.Close()
 
-	var locked sql.NullInt64
-	if err = conn.QueryRowContext(ctx,
-		"SELECT GET_LOCK("+snowflakeMigrationLock+", 60)").Scan(&locked); err != nil {
+	// PXC strict-mode clusters prohibit GET_LOCK; acquireMigrationLock falls back
+	// to lock-free execution in that case. migrateSnowflakePrimaryKey checks the
+	// current state of every table before touching it, so it is idempotent.
+	_, unlock, err := acquireMigrationLock(ctx, conn, snowflakeMigrationLock)
+	if err != nil {
 		return err
 	}
-	if !locked.Valid || locked.Int64 != 1 {
-		return errors.New("mysql: timed out waiting for snowflake primary-key migration lock")
+	if unlock != nil {
+		defer unlock()
 	}
-	defer conn.ExecContext(ctx, "SELECT RELEASE_LOCK("+snowflakeMigrationLock+")")
 
 	for _, spec := range snowflakePrimaryKeySpecs {
 		if err = migrateSnowflakePrimaryKey(ctx, conn, spec); err != nil {