feat(groups): show used traffic per group in groups table

Sum up+down across each group's clients via a LEFT JOIN on email in
ListGroups, expose it as trafficUsed on GroupSummary, and render it as a
new column plus a "Total traffic" summary card. Drops the unused "Empty
groups" card and its translation key.

web/translation/es-ES.json

@@ -812,7 +812,8 @@
       "clientCount": "Clientes en el grupo",
       "totalGroups": "Total de grupos",
       "totalGroupedClients": "Clientes con grupo",
-      "emptyGroups": "Grupos vacíos",
+      "trafficUsed": "Tráfico usado",
+      "totalTraffic": "Tráfico total",
       "addGroup": "Añadir grupo",
       "createSuccess": "Grupo «{name}» creado.",
       "rename": "Renombrar",