API stability pledge release

Given that multiple downstream applications now depend on the API contract, I think this is a good moment to seal it as stable and follow semver from there on. (I will remake it one day because it is Bad. But if I do, it’s not going to be at the cost of these existing applications – there would be a v1 endpoint and a v2 endpoint.)

Thanks to (in no particular order) @lynn, @robintown, @ixera, @leafpool243 for their contributions over the years.