Endpoint to get all subscriptions as an OPML

[tynanpurdy]

I don't know the technical feasibility but it would be nice if there were an endpoint that returned all my app.skyreader.feed.subscription records as one OPML file. NetNewsWire have indicated that they may be willing to do an ATproto integration if such an endpoint existed.

[disnet]

added OPML export in cc5633e but this is just on the frontend. should also have an API at some point for integrations like that