refactor: reorganize server modules

- Moved each individual service definition into a dedicated `services/`
  directory under every server module (e.g.
`modules/servers/bragi/services/`).
- Updated the corresponding `default.nix` files to import the renamed
  service modules from the new location.
- Applied the same changes across all server modules, ensuring
  the API and import paths remain consistent.

modules/servers/heimdall/wireguard.nix

@@ -1,24 +0,0 @@
-{config, ...}: {
-  networking = {
-    nat = {
-      enable = true;
-      enableIPv6 = true;
-      externalInterface = "enp1s0";
-      internalInterfaces = ["wg0"];
-    };
-    firewall = {
-      allowedTCPPorts = [53];
-      allowedUDPPorts = [53 51820];
-    };
-    wg-quick.interfaces.wg0.configFile = "${config.age.secrets.wg-heimdall.path}";
-  };
-
-  services.dnsmasq = {
-    enable = true;
-    settings = {
-      interface = "wg0";
-    };
-  };
-
-  boot.kernel.sysctl."net.ipv4.ip_forward" = 1;
-}