auth.sticknife.com moved to charon.sticknife.com. Rather than swapping the
hostname in each place, derive the OIDC issuer and the logout fallback from
AUTH_BASE_URL so the auth host is configured in one place.
Backend routes using charon.potternet.lan are unchanged; that internal name
remains the preferred route.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
ryan