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/id-ID.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "Grup",
       "name": "Nama",
-      "clientCount": "Klien di grup",
+      "clientCount": "Klien",
       "totalGroups": "Total grup",
       "totalGroupedClients": "Klien dengan grup",
       "trafficUsed": "Trafik terpakai",
+      "upload": "Unggah",
+      "download": "Unduh",
       "totalTraffic": "Total trafik",
       "addGroup": "Tambah grup",
       "createSuccess": "Grup «{name}» dibuat.",