fix(mqtt): 修复定期清理任务调度问题

- 将定时任务调度方式从 schedule 改为 scheduleWithFixedDelay
- 确保 flappingDetector 的 cleanup 方法能够按固定延迟重复执行
- 解决了过期记录清理功能可能无法正常工作的潜在问题

Author: smthing

plugins/enterprise-plugin/src/main/java/tech/smartboot/mqtt/plugin/EnterprisePlugin.java

@@ -103,7 +103,7 @@ public Thread newThread(Runnable r) {
             });
 
             // 定期清理过期记录
-            selfRescueTimer().schedule(flappingDetector::cleanup, 30, TimeUnit.SECONDS);
+            selfRescueTimer().scheduleWithFixedDelay(flappingDetector::cleanup, 30, TimeUnit.SECONDS);
         }
 
         log("企业版插件初始化完成，OpenAPI 服务监听地址: " + config.getHttp().getHost() + ":" + config.getHttp().getPort());