Start Assessment

One click on "start" from the home's next-best-action block.

Same core as the magic link's start endpoint, including the eligibility gate and the email-first rollback, so the two doors cannot disagree about what may be started. Concurrent assessments ARE allowed (a candidate is free to pivot from frontend to backend without abandoning anything); what both doors share is the in-flight budget, rule 6 of the eligibility module, which 409s with reason='at_cap'.