Tailscale's keys API has no separate expire verb: DELETE is the revoke.
Map it to a soft revoke so the key stays retrievable as invalid afterwards
instead of vanishing, matching the SDK and Terraform's expectations.
Add a revoked timestamp to pre-auth keys (migration plus schema), mark a
key invalid once revoked, and have the keys DELETE handler stamp it rather
than destroy the row. A background collector reaps revoked keys after a
configurable retention window (preauth_keys.revoked_retention, default
168h), so the table does not grow without bound.
Kristoffer Dalby
6b413fe237