The domain name has changed to a more beautiful one, https://jpapi.polv.cc. Still being hosted on free tier Heroku, but now hiding behind Cloudflare.
The only potential problem is the database is hosted on MongoDB Atlas, using 300 MB (they give 500 for free, anyway.)
So, the dependencies are not only Heroku, but also MongoDB Atlas.
I also made it easy for people to self-host by dumping the database; and upload the server image to Docker Hub. The database dump is on GitHub Release. All free for me, including the domain name.
Now, I am considering rate limiting the API as well. Will that be a problem for you, when you might have some traffic?