Task Onboard Creator

Run the full onboard_creator() pipeline for one user.

Triggered by a Cloud Task created in services.onboarding_dispatcher.enqueue_onboarding(), which fires from the OAuth callback at routes.dashboard_auth_routes.oauth_callback after the youtube_channels INSERT.

Always calls mark_onboarding_complete (success=True or False) so the safety-net row in onboarding_queue is cleared and the nightly cron does not re-run this user. If THIS endpoint fails entirely (raises before reaching mark_onboarding_complete), nightly-processing will pick up the still-pending row at 01:00 UTC.