Implement RFC 8414 for OAuth discovery

[Floppy]

I'd highly encourage you to also implement RFC 8414: https://datatracker.ietf.org/doc/html/rfc8414 You can see this in Mastodon here: https://github.com/mastodon/mastodon/blob/main/app/controllers/well_known/oauth_metadata_controller.rb There is work to lift that up into doorkeeper, but I haven't had time to get back to it (funding): https://github.com/doorkeeper-gem/doorkeeper/pull/1737

Originally posted by @ThisIsMissEm in https://github.com/manyfold3d/manyfold/issues/3951#issuecomment-2790433885