A creator's own hostname, so their newsletter lives at their address
rather than at ours. The same prove you own this hostname flow as Senders,
pointed at the website rather than at the mail. Needs settings: a
hostname is how the newsletter is configured, not how it sends.
Each one serves one of two things, which mode names. app is the
Commune newsletter page, with readers signed in; site is the creator's
own pages from Creator websites, on an origin that never carries a
session. Changing which is the one write here. Adding a hostname and
proving you own it happen in Commune itself, because both talk to the
certificate provider.