fix(nix-cache): use FQDN to fix hostname resolution on clients

systemd-resolved only uses LLMNR for single-label hostnames, never DNS —
same issue mount-data.nix already documented and fixed for NFS by switching
to server.sweet.home. Change the substituter URL, SSH knownHosts, and
remote-builder hostName from bare "nix-cache" to "nix-cache.sweet.home",
and update nginx's virtualHost to match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/nix-cache/remote-builder-client.nix

@@ -8,12 +8,12 @@
   # dedicated keypair). If this host doesn't have one yet:
   #   sudo -u root ssh-keygen -t ed25519 -N '' -f /root/.ssh/id_ed25519
   #   # then add its .pub to vars.remoteBuilderAuthorizedKeys and rebuild nix-cache
-  #   sudo ssh -i /root/.ssh/id_ed25519 nixremote@nix-cache nix-store --version
+  #   sudo ssh -i /root/.ssh/id_ed25519 nixremote@nix-cache.sweet.home nix-store --version
   # Trust nix-cache's SSH host key declaratively so the nix-daemon (root)
   # can connect the first time without a manual ssh-keyscan/known_hosts
   # step on every new client.
-  programs.ssh.knownHosts.${vars.nixCacheHost} = {
-    hostNames = [ vars.nixCacheHost ];
+  programs.ssh.knownHosts."${vars.nixCacheHost}.${vars.homeDomain}" = {
+    hostNames = [ "${vars.nixCacheHost}.${vars.homeDomain}" ];
     publicKey = vars.nixCacheHostKey;
   };
 
@@ -22,7 +22,7 @@
 
     buildMachines = [
       {
-        hostName = vars.nixCacheHost;
+        hostName = "${vars.nixCacheHost}.${vars.homeDomain}";
         sshUser = vars.remoteBuilderUser;
         sshKey = "/root/.ssh/id_ed25519";
         inherit (pkgs.stdenv.hostPlatform) system;