Export an Audience brief or validation report

Generates the Audience brief (kind: brief, default; pdf, md, docx, pptx) or the validation report (kind: validation_report; pdf, md, docx, xlsx): every scored validation with the overall score calculation, every KPI with its 95% interval, per-question answer shares, the composition check and provenance. The validation report needs edit access to the Audience and at least one scored validation (409 VALIDATION_REPORT_UNAVAILABLE otherwise). A cached artifact returns at once; otherwise the job is queued and polled through export-status.