After uploading, content thumbnails were blank until the item was added to a
playlist/widget. The public /api/content/:id/thumbnail (and /file) endpoints are
reference-gated (an anonymous player with a UUID must not pull arbitrary tenants'
media), and a plain <img> can't send a Bearer token - so a just-uploaded item 403'd.
- Backend: add an authenticated bypass - a logged-in user who can access the
content's workspace (verified from the Bearer token) may view its file/thumbnail
even when unreferenced. Anonymous players still hit the reference gate.
- Frontend: the content library lazy-fetches thumbnails/previews WITH the token
and swaps in an object URL (IntersectionObserver keeps it under the rate limit;
the URL is revoked after load).
Verified: unreferenced thumbnail now 200 with a bearer token, still 403 anonymous.