Complete Onboarding

Mark onboarding as complete and seed initial content — once.

Called by the mobile app after the user finishes the budget-onboarding step (bank connected + goals set + budgets set). Triggers first weekly briefing and challenge generation as background tasks so the home tab has content immediately.

Idempotent across replicas. The guard is a single atomic UPDATE on users.onboarding_completed_at (NULL -> now()); only the call whose UPDATE matches a row spawns the background work. There is no SELECT-then-UPDATE: with two replicas that is exactly the race that produced 8 briefing + 8 challenge model calls for one user on 17 Sep. The stamp is committed before any task is scheduled, so a task can never start for a call that will later lose the race. Repeat calls return the same 200 body and spawn nothing.

Both generators run independently — failure of either does NOT block the onboarding response.