Support fedilinks

[sneakers-the-rat]

Context

https://chaos.social/@SoniEx2/111691866484662475

so the main thing a website/host can do here is provide some sort of (atom) feed viewer for folks clicking web+feed links, as per https://fedilinks.org/2

(note that web+feed is atom-only. but anyway.)

another thing to consider is supporting web+feed throughout the software stack, so if someone types web+feed://soniex2.autistic.space/index.atom it should be able to parse that as a link, if it would also parse https as a link.

Should be straightforward - i assume FastAPI should support protocol handlers. if not then it's time for some PRs.