More accurate, steadier billing for API key requests

You can now count on accurate billing when you use Kollab models through an API key: if a request is interrupted partway, it is settled by the real amount of usage once the work has finished, rather than by an estimate.
Billing is now steadier when many requests run at the same time, so usage is not counted twice or missed during busy moments.
A settlement that is still waiting when Kollab updates or restarts is now kept safely and completed afterward, so interrupted requests are still billed correctly.