Okay, this is not technically "SQLite writes" in GitHub Pages, but that was the most compelling way to accurately say what it is.

It's SQLite writes, done in GitHub Actions, saved in a commit, pushed to the repo, which triggers Pages rebuild, which then serves the updates DB content!

The workflow that writes to the database? That's triggered from the GitHub pages page, when you submit the guestbook form.

So, yes, this "full stack app" is fully hosted on GitHub Pages/ Actions. :)