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/services/beszel-hub.nix

@@ -0,0 +1,52 @@
+{pkgs, ...}: {
+  systemd.services.beszel-hub = {
+    enable = true;
+    description = "Beszel agent";
+    after = ["network.target"];
+
+    serviceConfig = {
+      Type = "simple";
+      Restart = "always";
+      RestartSec = 3;
+      User = "beszel";
+      Group = "beszel";
+      WorkingDirectory = "/var/lib/beszel";
+    };
+
+    script = "${pkgs.beszel}/bin/beszel-hub serve --http '127.0.0.1:6789'";
+
+    wantedBy = ["multi-user.target"];
+  };
+
+  users = {
+    users.beszel = {
+      isSystemUser = true;
+      home = "/var/lib/beszel";
+      createHome = true;
+      group = "beszel";
+    };
+    groups.beszel = {};
+  };
+
+  services.traefik.dynamicConfigOptions.http = {
+    services.beszel.loadBalancer.servers = [
+      {
+        url = "http://localhost:6789";
+      }
+    ];
+
+    routers.beszel = {
+      rule = "Host(`beszel.cronyakatsuki.xyz`)";
+      tls = {
+        certResolver = "porkbun";
+      };
+      service = "beszel";
+      entrypoints = "websecure";
+    };
+  };
+
+  services.restic.backups = {
+    local.paths = ["/var/lib/beszel"];
+    server.paths = ["/var/lib/beszel"];
+  };
+}