Something that follows this pattern of a single file backend is PocketBase. I’ve used it on personal projects and, while not being 1.0 yet, it’s growing quickly
pocketbase has helped me get a few boutique websites and systems out so far with incredible speed and a joy that I rarely feel near web these days.
Though I should remain open-minded. An even simpler backend would be nicer (maybe). Also the AI-friendly approach looks promising.