swarm-controller: default matrixHomeserverUrl from swarm.matrix.gatewayHost

matrixHomeserverUrl re-derived gatewayHost's own default
(chat.${swarmDomain}) instead of reading gatewayHost itself, so a
deployment that pins gatewayHost away from that default (the exact
case hive-matrix.nix's own option doc describes) left the controller
calling a name nothing serves. Read matrix.gatewayHost directly,
mirroring hive-matrix.nix's ctlHomeserverUrl. Add module-eval cases
that pin gatewayHost and that null it out, alongside the existing
unpinned default case.

Closes #4757

nix/host-modules/swarm-controller.nix

@@ -92,6 +92,7 @@ let
 
   forgeCfg = config.services.hyperhive.swarm.forge;
   swarmDomain = config.services.hyperhive.swarm.domain;
+  matrixCfg = config.services.hyperhive.swarm.matrix;
 
   # The controller's forge client speaks TLS to `https://${forgeCfg.domain}`,
   # which the gateway serves with a leaf signed by the hive CA — a CA
@@ -558,10 +559,12 @@ in
 
     matrixHomeserverUrl = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
-      # The same `chat.<swarm domain>` ./hive-matrix.nix serves its vhost on
-      # (`gatewayHost`): a swarm runs one homeserver.
-      default = if swarmDomain == null then null else "https://chat.${swarmDomain}";
-      defaultText = lib.literalExpression ''"https://chat.''${services.hyperhive.swarm.domain}"'';
+      # `gatewayHost`'s own vhost, not a re-derivation of its default: a
+      # deployment that pins `gatewayHost` (e.g. to keep a name from before
+      # it moved under the swarm domain) still has one homeserver, and this
+      # option has to name the same one.
+      default = if matrixCfg.gatewayHost == null then null else "https://${matrixCfg.gatewayHost}";
+      defaultText = lib.literalExpression ''"https://''${services.hyperhive.swarm.matrix.gatewayHost}"'';
       example = "https://matrix.example.org";
       description = ''
         Client-server API base of the swarm's homeserver. The controller