refactor(3202): authelia declares its own vhost and dns name

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.

nix/host-modules/hive-gateway/default.nix

@@ -64,11 +64,17 @@ let
   svcCert = "${tlsDir}/swarm-services.pem";
   svcKey = "${tlsDir}/swarm-services-key.pem";
 
+  # Styled static error pages. Built once here and reached two ways:
+  # directly by ./vhosts.nix, and via the published kit by any service
+  # module that aims an `error_page` at one.
+  errorPages = import ./error-pages.nix { inherit pkgs; };
+
   # The vhost construction kit (listen set / per-name TLS attrs /
-  # security headers). Computed here, published as `cfg.lib` below, and
-  # handed to ./vhosts.nix **as the published value** — so the tree the
-  # gateway renders and the kit a service module gets are the same
-  # object by construction, not by two call sites agreeing.
+  # security headers / error pages). Computed here, published as
+  # `cfg.lib` below, and handed to ./vhosts.nix **as the published
+  # value** — so the tree the gateway renders and the kit a service
+  # module gets are the same object by construction, not by two call
+  # sites agreeing.
   vhostLib = import ./vhost-lib.nix {
     inherit
       lib
@@ -78,6 +84,7 @@ let
       svcCert
       svcKey
       swarmServiceDomains
+      errorPages
       ;
   };
 
@@ -86,6 +93,7 @@ let
     inherit
       lib
       cfg
+      errorPages
       matrixCfg
       autheliaCfg
       uiCfg
@@ -94,7 +102,6 @@ let
       dashboardDist
       swaggerUiTheme
       ;
-    errorPages = import ./error-pages.nix { inherit pkgs; };
   };
 in
 {
@@ -373,7 +380,6 @@ in
         cfg
         networkCfg
         matrixCfg
-        autheliaCfg
         uiCfg
         hyperhiveDomain
         ;