feat(groups): show upload/download breakdown in group traffic

Add per-group up/down to GroupSummary (backend + schema), surface them
as Upload/Download columns in the groups table, and fold upload/download
into the Total traffic summary card. Rename the group "Clients in group"
column to just "Clients" across all locales.

internal/web/translation/ru-RU.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "Группы",
       "name": "Имя",
-      "clientCount": "Клиентов в группе",
+      "clientCount": "Клиенты",
       "totalGroups": "Всего групп",
       "totalGroupedClients": "Клиенты с группой",
       "trafficUsed": "Использованный трафик",
+      "upload": "Отправлено",
+      "download": "Получено",
       "totalTraffic": "Общий трафик",
       "addGroup": "Добавить группу",
       "createSuccess": "Группа «{name}» создана.",