Sorting by started_at let a manually re-run old workflow jump above a
genuinely newer run: the re-run's started_at bumps to now, but its run
number never changes, so the Run column stopped reading strictly
descending. run_id is assigned once at creation and never changes across
attempts, so (run_id, attempt) descending is both correct and simpler --
it also handles a queued run (no started_at yet) for free, no datetime.max
special case needed.