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/views/ChatView.vue

@@ -58,6 +58,7 @@ function handleSSEEvent(event: ChatEvent) {
           sender_name: msg.sender_name,
           sender_avatar: '',
           is_self: isSelf,
+          is_read: false,
           content_type: msg.content_type as ChatMessage['content_type'],
           content: msg.content,
           attachment_urls: msg.attachment_urls || [],
@@ -76,6 +77,13 @@ function handleSSEEvent(event: ChatEvent) {
   if (event.type === 'conversation_updated' && event.conversation_id === conversationID.value) {
     loadConversation()
   }
+  if (event.type === 'conversation_read' && event.conversation_id === conversationID.value) {
+    // 对方已读：仅当本端仍有未获回执的自己消息时重载，刷新「已读」状态
+    const isSelfReader = event.reader_type === 'user' && event.reader_id === currentUserId
+    if (!isSelfReader && messages.value.some(m => m.is_self && !m.is_read)) {
+      loadMessages(false)
+    }
+  }
 }
 
 const { onEvent } = useChatSSE('user', '/api/chats/events')
@@ -305,6 +313,9 @@ function handleKeydown(e: KeyboardEvent) {
                 <ChatAttachmentImage v-for="url in item.attachment_urls" :key="url" :source="url" />
               </div>
               <span class="message-time">{{ formatDateMinute(item.created_at) }}</span>
+              <span v-if="item.is_self" class="read-status" :class="{ read: item.is_read }">
+                {{ item.is_read ? '已读' : '未读' }}
+              </span>
             </div>
           </template>
         </div>
@@ -538,6 +549,16 @@ function handleKeydown(e: KeyboardEvent) {
   font-size: 11px;
 }
 
+.read-status {
+  margin-top: 2px;
+  color: #a1a8b4;
+  font-size: 11px;
+}
+
+.read-status.read {
+  color: #67c23a;
+}
+
 .system-message {
   max-width: 80%;
   padding: 6px 12px;