Restructures RunOnce from per-channel inline processing to collect-sort-process:
Phase 1 — discover, persist (UpsertVideo), apply pre-filters (seen/manual/backoff)
and collect surviving candidates with their discovery position.
Phase 2 — sort candidates by published_at DESC, NULLS LAST, pos ASC tiebreak
so videos with no publish date never jump ahead of dated content.
Phase 3 — process in sorted order through the unchanged globalFetchGate.
Before (per-channel): chanA=[v-old, v-mid], chanB=[v-new, v-null]
→ [v-old, v-mid, v-new, v-null]
After (newest-first): [v-new, v-mid, v-old, v-null]
Same set of videos processed; only the order changes within a pass. All existing
behaviour is preserved: failure isolation, backoff skip, manual mode,
channel-unavailable, stats. In-memory sort; no new table or persisted queue.
The ordering is onboarding prioritisation — new users get summaries of their most
recent, relevant videos first; the back-catalogue fills in behind across subsequent
passes. Both this background batch and the foreground 'Try now' button honour the
shared globalFetchGate: rate limiting is respected, not evaded.