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

web/src/services/api.ts

@@ -125,7 +125,29 @@ export const getSessions = (params?: {
   if (params?.pageSize) search.set('pageSize', String(params.pageSize))
   return getList<Session>(`/sessions?${search}`)
 }
-export const getSession = (id: number) => get<{ session: Session; messages: Message[]; events: SessionEvent[]; pending_count: number }>(`/sessions/${id}`)
+export const getSession = (id: number) => get<{
+  session: Session
+  messages: Message[]
+  events: SessionEvent[]
+  pending_count: number
+  max_seq?: number
+}>(`/sessions/${id}`)
+
+/** 按 seq 增量拉取消息（重连 catch-up）：seq > after_seq */
+export interface SessionMessagesSync {
+  messages: Message[]
+  after_seq: number
+  max_seq: number
+  has_more: boolean
+}
+export const getSessionMessages = (id: number, afterSeq = 0, limit = 200) => {
+  const search = new URLSearchParams()
+  if (afterSeq > 0) search.set('after_seq', String(afterSeq))
+  if (limit > 0) search.set('limit', String(limit))
+  const qs = search.toString()
+  return get<SessionMessagesSync>(`/sessions/${id}/messages${qs ? `?${qs}` : ''}`)
+}
+
 export const assignSession = (id: number, agentId: number) => post(`/sessions/${id}/assign`, { agent_id: agentId })
 export const claimSession = (id: number) => post(`/sessions/${id}/assign`, {})
 export const transferSession = (id: number, agentId: number) => post(`/sessions/${id}/transfer`, { agent_id: agentId })