smugmug-cli backup: a dated upload (see crate::dated) of the
configured directories, repeated every interval, for running unattended
(e.g. in a container).
Uploading files into albums by the day they were taken:
<root folder>/YYYY/MM/YYYY-MM-DD (with YYYY-MM-DD (2), … past
SmugMug’s per-album limit). Used by upload without --album and by
backup.