FediBuzz webfinger backreference failure

Hi @astro@c3d2.social, I think I found a bug with FediBuzz and how it does its Webfinger responses.

In a nutshell, FediBuzz tag actors have preferredUsername set to tag-whatever, but the corresponding webfinger lookup ?resource=acct:tag-whatever@relay.fedi.buzz fails with a 404, but whatever@relay.fedi.buzz works fine.

cc @evan@cosocial.ca

AI slop explanation follows (feel free to ignore!):

> tags.pub does it correctly:
>
> json > { > "id": "https://tags.pub/user/nodebb", > "preferredUsername": "nodebb" > } >
>
> WebFinger acct:nodebb@tags.pub → resolves to https://tags.pub/user/nodebb
>
> FediBuzz breaks the round-trip:
>
> json > { > "id": "https://relay.fedi.buzz/tag/nodebb", > "preferredUsername": "tag-nodebb" > } >
>
> WebFinger acct:tag-nodebb@relay.fedi.buzz → 404
>
> Yes, it's a FediBuzz bug. preferredUsername must match the WebFinger subject so that acct:{preferredUsername}@{hostname} resolves back to the actor's id. FediBuzz added a tag- prefix to preferredUsername but didn't update WebFinger to match.
>
> They should either use preferredUsername: "nodebb" (matching tags.pub) or serve WebFinger for acct:tag-nodebb@relay.fedi.buzz.