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

---------

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

frontend/src/pages/clients/ClientInfoModal.tsx

@@ -218,7 +218,10 @@ export default function ClientInfoModal({
                     {client.enable && isOnline
                       ? <Tag color="green">{t('pages.clients.online')}</Tag>
                       : <Tag>{t('pages.clients.offline')}</Tag>}
-                    <span className="hint">{t('lastOnline')}: {dateLabel(traffic?.lastOnline)}</span>
+                    <span className="hint">
+                      {t('lastOnline')}: {dateLabel(traffic?.lastOnline)}
+                      {' · '}{t('lastSubFetch')}: {dateLabel(traffic?.lastSubFetch)}
+                    </span>
                   </td>
                 </tr>
                 <tr>