Get Tracking Status

The current status Roost holds for one tracking number.

ENSURES THE ROW EXISTS rather than 404ing: a number closed seconds ago, before tracking_status_worker's own discovery step next runs, must read as "not checked yet" (status "unknown", every timestamp null) rather than as an error -- and creating the row here is what makes the very next worker cycle pick it up first, per tracking_status.ensure_tracked's own docstring.