Stream a ZIP of all attendee face images for a project, named by person name.

Assembled on disk and streamed, not held in memory. It used to read every face image into RAM, build the archive in a BytesIO, and then copy the whole archive again into the response body — three copies of the same bytes. Face images are the full-size originals (~5MB each), so a project with a few hundred attendees exceeded the task's 2GB and took the container down with every other request on it. Now only one image is resident at a time.