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/tyr/nfs-server.nix

@@ -1,9 +0,0 @@
-{
-  services.nfs.server = {
-    enable = true;
-    exports = ''
-      /export/nfs     192.168.0.0/24(rw,sync,no_subtree_check) 172.16.0.0/24(rw,sync,no_subtree_check)
-    '';
-  };
-  networking.firewall.allowedTCPPorts = [2049];
-}