The `_` vhost was serving the hive's own surface, so every dashboard and
agent-UI request matched the default server rather than a named vhost --
and so did a request for any name at all, including a raw IP.
Split it: `_` keeps only `return 444`, and the hive surface moves to a
vhost named for the hive domain. `_` is `mkDefault` so an operator can
claim default_server themselves, plus an assertion for the case where
they add one without turning ours off -- nginx refuses to start on a
duplicate default_server and nixpkgs asserts nothing, so that would
otherwise surface as a gateway outage at rebuild time.
c32a9367e4