fix(meeting): Task 16 会议销毁路径资源清理专项

前端修复：
- _onRoomEnded 保留结束页数据，新增 exitEndedRoom() 动作
  room.vue onUnload 在 ENDED 状态调用，释放 currentRoom / participants / chatMessages 等 pinia state
- endMeeting() API 成功后立即触发 _onRoomEnded 本地兜底，不再依赖 room.ended 广播
  避免 API 成功但 WS 抖动导致 engine / timer / AudioContext 泄漏
- 新增 _pendingBroadcastTimers 集合收纳 _broadcastSelfState 的 setTimeout 句柄
  _cleanupMedia 统一 clearTimeout，避免会议结束后 stale 重试逻辑排队

后端修复：
- MeetingLifecycleService.OnRoomEnded 统一生命周期收尾
  取消 graceTimers / emptyTTLTimers + Pipeline DEL host_grace / empty_ttl / 两个 handling 锁 key
- EndRoom 事务提交后遍历 activesBefore 调 cleanupRoomRedisResidual
  批量 DEL 每个前任活跃成员的 resourceTrackKey + memberStateKey
  补调 lifecycleSvc.OnRoomEnded 清理自身 timer + keys
- HandleEmptyRoomExpired MarkEnded 后补调 cleanupRoomRedisResidual + 清 host_grace 残留

验证：go vet + go build + npm run build:h5 全部通过

Made-with: Cursor

frontend/src/pages/meeting/room.vue

@@ -602,6 +602,11 @@ onUnload(() => {
   if (meetingStore.isInMeeting && meetingStore.localState !== MEETING_LOCAL_STATE_LEAVING) {
     meetingStore.leave().catch(() => {})
   }
+  // Task 16 资源清理：若停留在"会议已结束"页面后物理返回，清空 pinia state
+  // 避免 currentRoom / participants / chatMessages 等大对象常驻内存
+  if (meetingStore.localState === MEETING_LOCAL_STATE_ENDED) {
+    meetingStore.exitEndedRoom()
+  }
 })
 </script>