Oban worker that purges one trashed storage library: its files (bytes
included, through the normal delete path), its folders, then the library
(PhoenixKit.Modules.Storage.Libraries.purge_library/1).
Queued when a user's libraries are trashed because the user is deleted,
and by the daily trash prune for libraries trashed longer ago than the
retention period. A library that is gone, or no longer trashed, is left
alone. A library on a user's own bucket whose objects could not all be
deleted is retried ({:error, :objects_remain}).