feat(sub): expose last subscription fetch time (#6217)

* feat(sub): expose last subscription fetch time

Record successful GET subscription fetches per client and surface the timestamp in the client API and UI.

* fix(frontend): include last subscription fetch in client traffic

* Update sub_fetch_test.go

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Co-authored-by: Hermes Agent <hermes-agent@localhost>

internal/web/translation/ru-RU.json

@@ -66,6 +66,7 @@
   "comment": "Комментарий",
   "success": "Успешно",
   "lastOnline": "Был(а) в сети",
+  "lastSubFetch": "Последнее получение подписки",
   "getVersion": "Узнать версию",
   "install": "Установка",
   "clients": "Клиенты",