Domain-level forward-auth needs its own external_host domain when the IdP core
and outpost are on different hosts. sso.hubris.network -> Caddy -> LAN outpost.
Includes the redirect_uris-regeneration gotcha. Carry the DNS record into
Technitium in DNS Phase 2.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>