Security review flagged a hardcoded JWT secret fallback. A booth machine
started without JWT_SECRET would have signed tokens with a publicly-known
default, letting anyone forge an admin token — defeating the local-auth
anti-fraud model.
- requireJwtSecret() refuses to start on a missing, <32-char, or placeholder
secret (no insecure default).
- Add sign.expiresIn: 8h so minted tokens expire (bound to a shift).
- Add apps/server/.env.example documenting JWT_SECRET + how to generate it.
Verified: refuses with no secret and with the old placeholder; boots and
serves /health with a valid `openssl rand -hex 32` secret.
