Product intent: new users get summaries of their newest videos fast while the
back-catalogue fills behind, within the shared rate gate. The background batch
currently processes in subscription/channel order, not newest-first — this spec
closes that gap (collect candidates, sort published_at DESC NULLS LAST, process
in order, gate unchanged). Also corrects the docs to describe Try-now as
onboarding prioritisation, explicitly removing the prior "looks organic to
YouTube" traffic-disguising framing — rate limiting is respected, not evaded.
