新增消息已读状态

复用 chat_participants.last_read_at 推导已读，无需新表/迁移。

- 后端：MessageDTO 增加 is_read，toMessageDTOs 按「全部已读」语义计算
  （所有其他参与者均已读才算已读，1对1 即对方已读）；MarkRead 成功后
  推送 conversation_read 事件（含 reader 与 read_at），NotifyConversation
  与 NotifyAllAdmins 双通道通知，使发送方/客服实时刷新已读
- 前端：ChatMessage 增加 is_read，ChatView/AdminChatsView/MobileChatView
  在自己的消息下渲染「已读/未读」；收到 conversation_read 且本端仍有
  未获回执的自己消息时才重载，避免无谓请求

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/features/chats/composables/useChatSSE.ts

@@ -16,9 +16,12 @@ export interface SSEMessage {
 }
 
 export interface ChatEvent {
-  type: 'new_message' | 'conversation_updated'
+  type: 'new_message' | 'conversation_updated' | 'conversation_read'
   conversation_id: number
   message?: SSEMessage
+  reader_type?: string
+  reader_id?: number
+  read_at?: string
 }
 
 type EventHandler = (event: ChatEvent) => void
@@ -69,6 +72,15 @@ export function useChatSSE(scope: AuthScope, endpoint: string) {
       }
     })
 
+    source.addEventListener('conversation_read', e => {
+      try {
+        const data = JSON.parse((e as MessageEvent).data) as ChatEvent
+        handlers.forEach(h => h(data))
+      } catch {
+        /* ignore */
+      }
+    })
+
     source.onerror = async () => {
       connected.value = false
       source?.close()