One-time (per re-record) admin upload of the demo video.

Deliberately an ENDPOINT and not only a deploy step: the asset store lives on a persistent volume that a developer's laptop does not have, so "copy the file onto the server" is not a thing anyone can actually do on a managed host. backend/scripts/upload_onboarding_demo.py drives this over HTTPS.

The body is streamed straight through to the store — nothing here reads the whole file — so a 500 MB upload costs one chunk of memory on the way past.