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/fa-IR.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "گروه‌ها",
       "name": "نام",
-      "clientCount": "کاربران در گروه",
+      "clientCount": "کاربران",
       "totalGroups": "تعداد گروه‌ها",
       "totalGroupedClients": "کاربران دارای گروه",
       "trafficUsed": "ترافیک مصرف‌شده",
+      "upload": "آپلود",
+      "download": "دانلود",
       "totalTraffic": "مجموع ترافیک",
       "addGroup": "افزودن گروه",
       "createSuccess": "گروه «{name}» ایجاد شد.",