Preview a business join link

Public (unauthenticated) preview of a business's shareable join link — the unguessable code in the URL is the credential. Returns the business, who shared the link and the role the visitor will land in. A code that is unknown, switched off, reset or expired all return the same 404, so a stranger cannot tell which. A client that keeps presenting codes that do not resolve is answered with 404 for everything for the rest of the hour.