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.
The webui bootstrap endpoint (/webui/bootstrap) rejected all non-localhost
connections with HTTP 403, preventing the embedded webui from working when
accessed from another device on the LAN — even when host was set to 0.0.0.0.
Skip the localhost check when the server is explicitly bound to 0.0.0.0 or ::,
since that signals intent to accept external connections.
