Traffic balance, expiry, credentials and purchase limits

One GB balance per organisation, usable on the mobile 4G/5G and the residential pool. credentials is null until the first purchase is fulfilled, and for an API key without the ports:write scope (the balance stays readable with ports:read). usage is synced from the gateway every 5 minutes. With limits.validity_days 0 GB never expire and expires_at is null; with a positive value every purchase extends expires_at to max(now, current expiry) + that many days.