Upload opt-in form proof

Hosts a screenshot (or PDF) of your SMS opt-in form and returns its public URL. Include that URL in the campaign's messageFlow (the opt-in workflow text) — the carrier registry has no attachment field, so reviewers verify consent by opening links in that answer. Works before a registration exists (use it when registering) and for appeals. /v1/sms/registrations/{id}/opt-in-proof is an alias.