fix: notify p2p message mutation on user channel

docs/IM-前后端对接文档.md

@@ -455,6 +455,7 @@ X-MeetingIM-APIKey: <预置的普通 API Key>
 | 修改好友备注 | 操作人本人 | `{"info":{"topic":"usr_actor","from":"usr_actor","what":"im","event":"friend.remark.updated","payload":{"user_id":"usr_actor"}}}` | 刷新好友列表和会话头显示名 |
 | 消息撤回 | Topic 在线成员 | `{"info":{"topic":"p2p_or_grp","from":"usr_actor","what":"im","event":"message.recalled","payload":{"topic":"p2p_or_grp","seq":12,"version":2}}}` | 按 `topic+seq+version` 重新拉取或本地标记已撤回 |
 | 消息编辑 | Topic 在线成员 | `{"info":{"topic":"p2p_or_grp","from":"usr_actor","what":"im","event":"message.edited","payload":{"topic":"p2p_or_grp","seq":12,"version":2}}}` | 按 `topic+seq+version` 重新拉取消息详情或修订记录 |
+| 单聊消息变更脏标记 | P2P 双方在线会话 | `{"info":{"topic":"usr_target","from":"usr_actor","what":"im","event":"message.mutation","payload":{"topic":"p2p...","seq":12,"version":2,"event":"message.recalled"}}}` | 当前未订阅该 P2P 也要标记会话脏，并调用 `message-mutations` 做补偿同步 |
 | 群业务变化 | 群 Topic 在线成员 | `{"info":{"topic":"grp...","from":"usr_actor","what":"group","event":"group.member.joined","payload":{"topic":"grp...","user_id":"usr_xxx"}}}` | 按事件刷新群详情、成员列表、入群申请、公告、禁言或角色权限；除操作人外当前会写系统通知 |
 | 会议变化 | 会议参会人 | `{"info":{"topic":"usr_target","from":"usr_actor","what":"im","event":"meeting.updated","payload":{"meeting_id":"mtg_1","version":2,"state":"scheduled"}}}` | 刷新会议列表/详情；除操作者外当前会写系统通知 |
 | 安全风险确认 | 操作人本人 | `{"info":{"topic":"usr_actor","from":"usr_actor","what":"im","event":"security.risk.continue","payload":{"risk_challenge_id":"risk_1","decision":"continue"}}}` | 刷新安全验证页面；当前会写系统通知 |
@@ -1590,7 +1591,32 @@ Body：
 
 #### 4.21.3 消息变更同步水位
 
-撤回、编辑属于 `seq` 不变的原地变更。在线且已订阅 Topic 时，客户端会收到 `message.recalled` / `message.edited` 实时事件；离线或未订阅时，需要靠会话级水位补偿。
+撤回、编辑属于 `seq` 不变的原地变更。在线且已订阅 Topic 时，客户端会收到 `message.recalled` / `message.edited` 实时事件；单聊中如果客户端在线但当前没有订阅该 P2P，服务端会额外向双方用户通道下发 `message.mutation` 脏标记事件；离线或错过实时事件时，需要靠会话级水位补偿。
+
+单聊未订阅脏标记示例：
+
+```json
+{
+  "info": {
+    "topic": "usr_target",
+    "from": "usr_actor",
+    "what": "im",
+    "event": "message.mutation",
+    "payload": {
+      "topic": "p2pJaOWxMvALjB4i1K_h_NZgQ",
+      "seq": 12,
+      "version": 2,
+      "event": "message.recalled"
+    }
+  }
+}
+```
+
+收到 `message.mutation` 后不要直接改本地消息内容，先把 `payload.topic` 标记为脏，再调用：
+
+```http
+GET /v1/im/topics/{topic}/message-mutations?since_version={localMutationVersion}&limit=50
+```
 
 会话列表同步时，`me.meta.sub[].private` 会带：