perf(chat): 优化后台客服会话查询与状态

backend/internal/modules/chat/message.go

@@ -40,12 +40,15 @@ func (r *Repository) Messages(ctx context.Context, principal Principal, conversa
 
 	offset := (page - 1) * pageSize
 	var rows []model.ChatMessage
-	if err := query.Order("id ASC").
+	if err := query.Order("id DESC").
 		Offset(offset).
 		Limit(pageSize).
 		Find(&rows).Error; err != nil {
 		return nil, err
 	}
+	for left, right := 0, len(rows)-1; left < right; left, right = left+1, right-1 {
+		rows[left], rows[right] = rows[right], rows[left]
+	}
 	items, err := r.toMessageDTOs(ctx, principal, rows)
 	if err != nil {
 		return nil, err
@@ -166,21 +169,44 @@ func (r *Repository) MarkRead(ctx context.Context, principal Principal, conversa
 	now := time.Now()
 	updated := false
 	err := r.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
-		// 管理员可以不是 participant，直接返回成功
 		if principal.Type == "admin" {
-			// 尝试查找 participant 记录，如果有就更新
-			var participant model.ChatParticipant
-			err := tx.Where("conversation_id = ? AND participant_type = ? AND participant_id = ?",
-				conversationID, principal.Type, principal.ID).First(&participant).Error
-			if err == nil {
-				// 有 participant 记录，更新已读时间
-				updated = true
-				return tx.Model(&participant).Update("last_read_at", now).Error
-			} else if errors.Is(err, gorm.ErrRecordNotFound) {
-				// 没有 participant 记录，直接返回成功（管理员无需记录已读）
+			var conversation model.ChatConversation
+			if err := tx.Select("id", "last_message_id").First(&conversation, conversationID).Error; err != nil {
+				if errors.Is(err, gorm.ErrRecordNotFound) {
+					return ErrConversationNotFound
+				}
+				return err
+			}
+			lastMessageID := uint64(0)
+			if conversation.LastMessageID != nil {
+				lastMessageID = *conversation.LastMessageID
+			}
+			var existing model.ChatAdminConversationState
+			err := tx.Where("conversation_id = ? AND admin_user_id = ?", conversationID, principal.ID).
+				First(&existing).Error
+			if err == nil && existing.LastReadMessageID >= lastMessageID {
 				return nil
 			}
-			return err
+			if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
+				return err
+			}
+			state := model.ChatAdminConversationState{
+				ConversationID:    conversationID,
+				AdminUserID:       principal.ID,
+				LastReadMessageID: lastMessageID,
+				LastReadAt:        &now,
+			}
+			if err := tx.Clauses(clause.OnConflict{
+				Columns: []clause.Column{{Name: "conversation_id"}, {Name: "admin_user_id"}},
+				DoUpdates: clause.Assignments(map[string]interface{}{
+					"last_read_message_id": lastMessageID,
+					"last_read_at":         now,
+				}),
+			}).Create(&state).Error; err != nil {
+				return err
+			}
+			updated = true
+			return nil
 		}
 
 		// 普通用户必须是 participant