Scans all leases once and revokes any that have expired. Returns the
number successfully reaped; individual failures are logged and skipped
(they’ll be retried on the next pass).
Revokes one lease by asking its owning engine mount to revoke() it,
then removing the lease record. Best-effort: engine failures are
reported to the caller but the lease record is only deleted on success,
so a failed revoke is retried on the next pass rather than silently lost.