I was investigating Durable Objects (DO) and had Fable walk me through where in my app they might be appropriate. One place had a dependency with billing (where I use a transaction now) and the proposed re-work to allow for concurrent editing with DO looked very much like this, reservations with idempotency keys. And if you add hierarchical allotments then it scales pretty well.

I disagree with the other posters about the bg process, if you have any bg processing already you should be able to handle the few edge cases without too much trouble.