r4 backup <channel> should download json, just as the app interface allows

In order to make the "cross-app experience more isomorphic", it would be nice if r4 backup <channel> could download the json backup of a channel (or at least a user's if logged in).

With the new /settings/import/backup

Currently r4 dl <channel> creates the folders <channel>/tracks/, so somehow either:

• r4 dl would also download a backup (would work for anon users?)
• r4 backup would check/create the <channel>/backup-date.json folder & file?

Questions/Issues:

• dl and backup commands don't have the same "timeline", while backup is almost instant (json), dl could takes while (files); therefore a backup.json file could very well "not be in sync" with what the tracks are? Not sure it is an issue, better always have a fresh dl (at the right place, with correct naming), whatever the command that is run?
• maybe we can have backup (just for json), and dl (calls backup and dl files)?

Start by reading the implementations of the r4 backup and r4 dl commands, then compare their behavior with /settings/import/backup. Resolve whether backup should remain JSON-only or whether dl should invoke it, including naming, directory creation, synchronization, and anonymous-user behavior; done means the chosen command produces the documented backup and track layout.