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/migration_lock.go

@@ -0,0 +1,36 @@
+//go:build mysql
+// +build mysql
+
+package mysql
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+
+	"github.com/jmoiron/sqlx"
+)
+
+// acquireMigrationLock wraps SELECT GET_LOCK so that schema migrations keep
+// working on Percona XtraDB Cluster where pxc_strict_mode=ENFORCING prohibits
+// named locks: instead of failing, locked=false is returned and the caller
+// proceeds without the lock. This is safe because Galera's total order
+// isolation serializes DDL across the cluster, and every migration using this
+// helper is idempotent: steps are guarded by existence checks, so concurrent
+// runs at worst fail with duplicate-key errors and are retried on the next
+// startup.
+func acquireMigrationLock(ctx context.Context, conn *sqlx.Conn, lockExpr string) (locked bool, unlock func(), err error) {
+	var held sql.NullInt64
+	if err = conn.QueryRowContext(ctx, "SELECT GET_LOCK("+lockExpr+", 60)").Scan(&held); err != nil {
+		if isPXCStrictModeNamedLockError(err) {
+			return false, nil, nil
+		}
+		return false, nil, err
+	}
+	if !held.Valid || held.Int64 != 1 {
+		return false, nil, errors.New("mysql: timed out waiting for schema migration lock")
+	}
+	return true, func() {
+		conn.ExecContext(ctx, "SELECT RELEASE_LOCK("+lockExpr+")")
+	}, nil
+}