Expedite purge on a trashed library

Expedites the background purge on a trashed library: the 90-day undo window is waived and the drain begins claiming this library on the next scheduled tick. Returns immediately; the drain proceeds asynchronously in bounded batches and does not block on completion. restore_library still works until the drain finishes purging all assets, but past this point it will recover only the assets the drain hasn't gotten to yet. Returns 409 if the library has not been trashed yet — call trash_library first.