DEPLOY GATE: Railway's healthcheck (#61). 503 ONLY when this build is proven OLDER than the database schema and no valid override names it — the 2026-09-29 image would have failed here instead of serving for 18 hours.

Never exits and never raises: a refused build just never goes live, the previous deployment keeps serving, and restartPolicyMaxRetries is never involved. Railway stops reading this once a deploy is live, so a schema that moves ahead later does not take this one down. checking is 503 only for DEPLOYZ_CHECKING_GRACE_S; unknown and newer are 200.