I’m not an expert here, but my initial assumption is that a lot of this could be done via regular checkpoints that are put on IPFS. There would be a server that takes in data from the Discourse/GitHub API, then runs the SourceCred algorithm on it, then creates checkpoints on that data in regular intervals. Then a “Cred Explorer” front-end could be built for end users to find data they want. It would be like a block explorer for a blockchain, but for SourceCred - and instead of data coming from a blockchain it would come from Discourse, GitHub, etc…
The “backend” here could be the “Cred Explorer.” It could track the history of any SourceCred instance over time (in this case SourceCred core) and send compute relevant data. When users boost a Thread, Initiative, or Artifact that data would get fed to the Cred Explorer. Boosting could be accomplished via a special emoji (like this one 
) and that could even be made possible as a reaction (like likes, but with other symbols) via the Retort plugin. We can currently edit Initiatives and Artifacts via Discourse wikis and the Initiatives plugin is being made to support that.
Seems like for an MVP we could have a Cred Explorer to run the SourceCred protocol and provide a nice UI to view the graph, but the game itself could be played via channels like GitHub, Discourse, and whatever else we integrate. This way we can focus on designing, testing, and validating the core game mechanics before anything else.
Also might be incorrect here as to what is and is not technically possible, but this is my initial impression.