Subs are easy; you just cache basic data for the listings and then request it from the external site when you need to view it. Any content on there is stored on the external site, and maybe a copy's kept for convenience on user pages but that's non-canonical.

Does SQLAlchemy let you add a new attribute to the magic Table object, run the code in prod and automatically update the database? Or do you have to run SQL commands to add the column manually?

Hadn't seen Lemmy; that's cool. Also, it reminds me of one other issue:

Other sites have different voting systems. If we're going to federate with them, we need to genericise the way we represent votes or everything will fall over when we try to pass content to and from somewhere with only one vote, or with upvote and downvote, or even with three types of vote.

Here's one possible way of doing that: we have a votes table, where each vote has an id. Then we— wait, that won't work.