Jetty is a servlet container and not really a general purpose web server.
Honestly the most straightforward thing to do would be to put your Jetty application behind a reverse proxy like nginx, Apache httpd or haproxy. From there you have the option of using the common tools and configurations like Certbot.
Otherwise, you will need to dive into the Jetty XML configuration files and figure out how to direct a the
/.well-known/acme-challenge context path to somewhere on your filesystem. This could be very tricky if the Jetty runtime is embedded into the Subsonic application (you’d need to edit fies inside
Finally you also have the solution to use the DNS challenge, but you may find that it is the most complicated of all, AND you’d still have to modify the Jetty XML files to use your certificate. It’s almost certainly a giant waste of time.