manatee: Add services homepage with auto-discovery

Add a homepage module that generates a static landing page served on
port 80 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/immich/default.nix

@@ -10,6 +10,12 @@ in
   };
 
   config = lib.mkIf enabled {
+    mod.homepage.services = [{
+      name = "Immich";
+      port = 2283;
+      description = "Photo library";
+    }];
+
     users.users.immich = {
       isSystemUser = true;
       group = "storage";