tadpole: Assert that nginx is running when using ppp.pm-site

hosts/tadpole/modules/ppp.pm-site/default.nix

@@ -6,8 +6,6 @@
 }:
 let
   enabled = config.mod.pppdotpm-site.enable;
-
-  nginxEnabled = config.mod.nginx.enable;
 in
 {
   imports = [ inputs.pppdotpm-site.nixosModules.default ];
@@ -18,7 +16,14 @@ in
     };
   };
 
-  config = lib.mkIf (enabled && nginxEnabled) {
+  config = lib.mkIf enabled {
+    assertions = [
+      {
+        assertion = config.services.nginx.enable;
+        message = "Option 'config.services.nginx' must be enabled";
+      }
+    ];
+
     services.pppdotpm-site = {
       enable = true;
       domain = "ppp.pm";