Kick a draft-dashboard PDF export

Start an on-demand async export of the editor's current (DRAFT) canvas to a PDF via dash-render. Returns a job_id; poll /jobs/{id} for {stage, progress} then the resolved {bytes, filename, sequence}, and GET /{id}/pdf/{job_id} for the bytes. The canvas version is pinned at submit and the client-hydrated data map is stashed server-side (kept off the Celery args — it can be MBs). Async because a synchronous Chromium print can outlive the proxy / client timeouts held on the request. Auth mirrors /data (author or viewer).