fix(observability): 治理SSE慢请求虚报、客户端取消日志污染、退款重试刷屏与会话计数慢查询

backend/internal/modules/chat/support.go

@@ -229,6 +229,7 @@ func (r *Repository) listAdminConversations(ctx context.Context, principal Princ
 	var rows []conversationRow
 	offset := (page - 1) * pageSize
 	queryDB := r.adminConversationBase(ctx, principal, keyword).
+		Joins("LEFT JOIN chat_messages AS lm ON lm.id = c.last_message_id").
 		Select(`c.id, c.order_id, c.listing_id, c.type, c.support_scene, c.title, c.status, c.last_message_id,
 			c.last_message_preview, c.last_message_at, c.created_at, c.updated_at,
 			COALESCE(cp_me.role, 'admin') AS role,
@@ -302,7 +303,6 @@ func (r *Repository) adminConversationBase(ctx context.Context, principal Princi
 	db := r.db.WithContext(ctx).Table("chat_conversations AS c").
 		Joins("LEFT JOIN chat_participants AS cp_me ON cp_me.conversation_id = c.id AND cp_me.participant_type = ? AND cp_me.participant_id = ?", principal.Type, principal.ID).
 		Joins("LEFT JOIN chat_admin_conversation_states AS cas ON cas.conversation_id = c.id AND cas.admin_user_id = ?", principal.ID).
-		Joins("LEFT JOIN chat_messages AS lm ON lm.id = c.last_message_id").
 		Joins("LEFT JOIN rental_orders AS explicit_lo ON explicit_lo.id = c.order_id").
 		Joins("LEFT JOIN rental_listings AS l ON l.id = COALESCE(explicit_lo.listing_id, c.listing_id)")
 	if keyword != "" {