Implement a /backup command

[zeel01]

Leo currently does database backups on start, but can't do one while running. This is due to the way the database driver is handled between using the sequelize ORM (which, at least last I checked, can't run a direct backup procedure) vs. better-sqlite3 which can.

In order to perform a backup while running, other subsystems that touch the database would need to be suspended, the sequelize DB connection will need to be ended, the backup performed using better-sqlite3, then the original connection restored and other systems re-enabled.

Or... something totally different that I didn't think of or using new features of things I haven't looked at in years.

Also, obviously this needs appropriate permissions handling.