Currently, there is no real developer documentation, and in fact there is no documentation at all besides (questionable?) tutorials. This could definitely be improved:
A swagger could be provided, however, the voting API is, to my knowledge, just one endpoint. (We would also host an instance of Swagger UI to display this swagger definition)
A custom page could be created as a means of API spec.
Additionally, @Nouish requested that we add the voter's IP to the callback. This could be used to verify legitimacy of the vote.
If there are more suggestions please post them below and I will add them to the topic post.
The forum is not self-hosted, but is hosted by IPS.
I'd have to ask them to enable it for us. Most likely won't be an issue, but I'd rather do it after my vacation and preferably also after high season. I don't want the site to end up with possible Mixed Content errors while I'm unavailable :-)