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/es-ES.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "Grupos",
       "name": "Nombre",
-      "clientCount": "Clientes en el grupo",
+      "clientCount": "Clientes",
       "totalGroups": "Total de grupos",
       "totalGroupedClients": "Clientes con grupo",
       "trafficUsed": "Tráfico usado",
+      "upload": "Subida",
+      "download": "Bajada",
       "totalTraffic": "Tráfico total",
       "addGroup": "Añadir grupo",
       "createSuccess": "Grupo «{name}» creado.",