Attach a document the dealer sent us outside the questionnaire.

The case this exists for: onboarding is finished, the questionnaire is closed, and the certidão permanente arrives by email weeks later. Without this the only way to file it is to reopen a completed questionnaire and wait for the client to re-upload — a state transition and a round trip to record a PDF somebody already sent us.

Rate limited per USER, not per IP: signed-in traffic reaches us through a handful of CloudFront edge addresses, so an IP bucket here would throttle unrelated tenants together (services/rate_limit.py says so explicitly).

⚠️ This route MUST stay under the /api/admin/onboarding-questionnaires prefix. That prefix is in the audit middleware's _BODY_OMIT_PREFIXES, so the middleware never calls await request.body() on it — which is what keeps a 10MB PDF out of both the audit table and the middleware's buffer. It is the only multipart POST on the audited /api surface.

Ambient-tenant rule: the company is whatever the loaded questionnaire row says, never user["idcompany"].