Group intros were stamped as posted before the outbox drain ran. When
delivery enqueue failed or the intro row exhausted retries, the channel
stayed marked and no retry path existed — unlinked participants never saw
the onboarding message.
Stamp introPostedAt in the drain path after a successful provider send,
matching the messagingMirroredAt pattern from #42.
Co-authored-by: cynarAI <cynarAI@users.noreply.github.com>
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