manatee: Add services homepage with auto-discovery

Add a homepage module that generates a static landing page served on
port 9999 via nginx. Each service module registers itself via the shared
mod.homepage.services option, so enabling a module automatically adds
it to the page.

hosts/manatee/modules/syncthing/default.nix

@@ -10,6 +10,12 @@ in
   };
 
   config = lib.mkIf enabled {
+    mod.homepage.services = [{
+      name = "Syncthing";
+      port = 8384;
+      description = "File sync";
+    }];
+
     services.syncthing = {
       enable = true;