优化实时聊天：稳定 WS、按 seq 增量同步，并修复访客评价推送

- 客服/访客 WebSocket 不再因切换会话反复重连，断线自动恢复
- 新增 after_seq 增量拉取，重连与消息空洞时 catch-up
- 结束会话同时推送给访客，预览页可弹出评价并支持兜底状态同步

server/internal/handler/router.go

@@ -61,6 +61,7 @@ func SetupRoutes(r *gin.Engine, store storage.ObjectStorage, storageCfg config.S
 		sessions.POST("/:id/end", session.End)
 		sessions.POST("/:id/archive", session.Archive)
 		sessions.PUT("/:id/priority", session.UpdatePriority)
+		sessions.GET("/:id/messages", session.ListMessages)
 		sessions.POST("/:id/messages", session.SendMessage)
 		sessions.POST("/:id/read", session.MarkRead)
 		sessions.POST("/:id/notes", session.AddNote)