Aria2[https://github.com/aria2/aria2] is a torrent, and direct download handler.
It runs on a host much like nzbget/sonarr.
It has a full featured API and existing clients do exist in the form of web. And some terrible android ones.
That said I would petition for this to be added. It uses a JSONRPC interface api just like Nzbget, Transmission Deluge and many others.
JSONRPC Interface Manual: https://aria2.github.io/manual/en/html/ ... -interface