When a thumbnail is requested but not yet available, attempt to generate
it synchronously on-demand instead of immediately falling back to the
placeholder image.
- Export thumbnail generation helpers from the jobs package.
- Update the Thumbnail handler to trigger on-demand generation if the
thumbnail object is missing.
- Save the updated box metadata with the new thumbnail reference.
- Fall back to the placeholder image only if on-demand generation fails.
Daniel Legt
eff831b142