Function serve 

pub async fn serve(
    sites: Vec<Site>,
    client: Client,
    config: AppConfig,
) -> Result<()>

Run the server until the listener closes or a shutdown signal arrives.

sites is the pre-filtered site list every scan dispatched through this server runs against. client is the pre-built HTTP client (so configuration like proxy, throttle, and browser backend flows from the CLI flags through here unchanged).