The published version as a PDF download

The latest published version's PDF, rendered once and kept in object storage, since a published version never changes (spec §7.2). Report-engine drafts print every figure, table and chart (as its data table). 404 when REPORTS_V2_ENABLED is off or nothing is published. Needs report.export_pdf. The earlier POST /reports/{id}/export/pdf (base64 in JSON) is unchanged.