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/user_identity_schema.go

@@ -5,7 +5,6 @@ package mysql
 
 import (
 	"context"
-	"database/sql"
 	"errors"
 	"fmt"
 	"strings"
@@ -32,14 +31,35 @@ func ensureUserIdentitySchema(db *sqlx.DB) error {
 	}
 	defer conn.Close()
 
-	var locked sql.NullInt64
-	if err = conn.QueryRowContext(ctx, "SELECT GET_LOCK("+userIdentityMigrationLock+", 60)").Scan(&locked); err != nil {
+	// Fast path: the last two steps of this migration drop the legacy im_usertags
+	// table and the im_users.tags column. If both are already gone, the migration
+	// has fully run (or the database was created fresh) and no lock is needed.
+	stale := false
+	if exists, err := identityTableExists(ctx, conn, "im_usertags"); err != nil {
+		return err
+	} else if exists {
+		stale = true
+	}
+	if !stale {
+		if exists, err := identityColumnExists(ctx, conn, "im_users", "tags"); err != nil {
+			return err
+		} else if exists {
+			stale = true
+		}
+	}
+	if !stale {
+		return nil
+	}
+
+	// PXC strict-mode clusters prohibit GET_LOCK; acquireMigrationLock falls back
+	// to lock-free execution in that case. The migration body is idempotent.
+	_, unlock, err := acquireMigrationLock(ctx, conn, userIdentityMigrationLock)
+	if err != nil {
 		return err
 	}
-	if !locked.Valid || locked.Int64 != 1 {
-		return errors.New("mysql: timed out waiting for user identity migration lock")
+	if unlock != nil {
+		defer unlock()
 	}
-	defer conn.ExecContext(ctx, "SELECT RELEASE_LOCK("+userIdentityMigrationLock+")")
 
 	usersExist, err := identityTableExists(ctx, conn, "im_users")
 	if err != nil || !usersExist {