A failed post returns retry(), which moves the next run onto
WorkManager's exponential backoff — 30s doubling, capped at five hours.
A couple of failures while the claw is unreachable is enough to leave
the fleet without a report for most of a day, which reads exactly like
the phone being gone.
The snapshot is only saved when every post landed, so the next
scheduled run re-derives the whole transition set and posts it again.
The fifteen-minute schedule is the retry; return success and keep it.