Looks like they don't accept signatures with algorithm=hs2019
Now that that's resolved, there's still another issue with Signature parsing: https://github.com/friendica/friendica/issues/8847
Looks like Friendica sends posts with titles as Article, not sure how we want to handle that.
For now I'll make it work the same as Page
Found some issues with comment targeting, replying to Friendica posts seems to work now
Friendica's Announces seem to be fixed now, still doesn't accept our posts though