Replace the == string comparison at delivery.py:127 with
hmac.compare_digest so the shared secret never leaks through response
timing (repo convention: stripe_client.py:88, server.py:171). The
isinstance/empty guards run first so compare_digest's TypeError on
non-str input is never reached — unset config, None, and non-str keys
stay fail-closed False. Five direct authorized() tests added (16
delivery tests / 42 full suite green).