Two parallel tabs both reading `onboardingStep = 2` and both POSTing
`step: 3` would both succeed because the second write didn't re-check
the precondition — the wizard could double-advance and skip a step.
Move the write onto `updateMany` with the precondition embedded in
the WHERE clause: exactly one update lands, the loser sees count = 0
and returns 409 so the client can refresh and retry.
The WHERE clause covers `onboardingCompletedAt: null` so the same
guard catches a race with `POST /api/onboarding/complete`. The
`current === 0` branch widens the match to `step IS NULL OR step = 0`
so legacy pre-W14b accounts still advance from step 1.
Fix-M / code-M7.
Marc-André Bombeck
7c9b4b6563