Seed the effective domain from the request's own hostname.

loadEffectiveDomain() runs late in the request (after settings are loaded), so anything that fails before it — most notably database migrations on the first request after a cold boot — would otherwise read the bare "localhost" fallback in error notifications. Seeding the request host early makes those notifications (e.g. ntfy titles) identify the real site. The value is refined later by loadEffectiveDomain() once the custom domain is known.