Make payment server answer with a 404 by default

[Copy pasta](https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/blob/ca44204076983ce02fd3e0dacd940514e0821e23/nixos/modules/spending.nix#L130) - Thanks @tomprince!

nixos/modules/issuer.nix

@@ -224,6 +224,12 @@ in {
           '';
           proxyPass = "http://127.0.0.1:${internalHttpPort}";
         };
+        locations."/" = {
+          # Return a 404 error for any paths not specified above.
+          extraConfig = ''
+            return 404;
+          '';
+        };
       };
     };