A 503 from the issuer's discovery endpoint made `serve` exit and the pod
restart. New now retries discovery 5 times (2+4+8+16 s back-off, ~30 s)
before failing as before; a cancelled ctx ends the back-off at once.
WithDiscoveryRetry lets tests shrink the schedule.
Closes#59
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
