_template_of_backup only stripped one "_"-chunk off the archive name,
so real Template_<name>_%Y%m%d_%H%M%S.7z files (date *and* time) never
matched their template -- backups silently failed to show up grouped
under it. Replaced with a regex that strips the full optional
date+time suffix and tolerates underscores in template names.
Also adds delete_artifact/delete_backup (mirrored, containment-checked
single-file deletion) and backups_by_template (groups list_backups()
output by parsed template name, newest-first) for the web dashboard's
per-item delete actions and the Templates panel's per-row backup list.
Simone
4541dccf9c