Moves the authelia vhost out of the gateway's vhosts.nix and its
`address=` rule out of dnsmasq.nix, into swarm-authelia.nix.
Both land inside that module's existing `cfg.enable` guard, which is
the load-bearing part: every hive in a swarm knows `authelia.url`, but
only the host that RUNS the container may claim the name. A client hive
declaring the vhost would answer for a service it does not run, and
publishing the DNS record would point every agent on its bridge at that
wrong answer.
The kit grows a fourth member, `errorPages`, because the vhost aims its
502/503/504 at the gateway's styled sso-unavailable page. Republished
rather than imported per module: a service rendering its own would drift
from the rest of the gateway the first time the theme changed.
Slice 1 of #3202. The listen set, the per-name TLS attrs and the
security headers move out of vhosts.nix into ./vhost-lib.nix and are
published as `services.hyperhive.gateway.lib` (internal, readOnly).
No behaviour change: vhosts.nix consumes the published value, so the
rendered vhost tree is identical.
The point is the next slice. Today a swarm service's vhost lives in
the gateway because only the gateway knows the port pair, the issuer
for a name, and the header block. Publishing those three is what lets
a service module declare its own vhost without the gateway having to
know that service by name.
