The previous LAN-access fix (PR #3656) relaxed the bootstrap localhost
check when host was 0.0.0.0, but did not require any authentication —
any device on the network could obtain a token without credentials.
New behavior:
- token_issue_secret configured: always validate, regardless of source
IP (handles reverse-proxy scenarios where all connections appear as
localhost).
- No secret configured: only localhost can bootstrap (local dev mode).
This supersedes the host-based check from PR #3656.