The Oban job that starts the purge of one trashed storage library
(Storage.Jobs.PurgeLibrary, a job run): 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 — inside that transaction, which is why this is a job and not a direct start (the engine refuses to start a run inside a caller's transaction) — and by the daily trash prune for libraries trashed longer ago than the retention period. A library that is gone, or no longer trashed, ends the run with nothing done. One that could not be fully deleted from a user's own bucket is retried by the run.