Basically, one of the backend maintenance tasks for compute servers should be to check periodically
for compute servers that are associated with a DELETED project. After X days, any such compute
servers should be deprovisioned.
Regarding defining how long since project was deleted, instead use the last_edited field of the project.
This is backend functionality that would be part of "maintenance" and be complementary to https://github.com/sagemathinc/cocalc/issues/7520
Basically, one of the backend maintenance tasks for compute servers should be to check periodically for compute servers that are associated with a DELETED project. After X days, any such compute servers should be deprovisioned.
Regarding defining how long since project was deleted, instead use the last_edited field of the project.