Presign the previews for many parts in ONE request.

A list view needs a thumbnail per row, and the per-part endpoint made that one HTTP call per row: a 50-row page fired 35 of them, which on the Free tier (60 weighted calls/minute) exhausted the whole budget on a single page load — the tail 429'd, the thumbnails broke, and the client's expired-URL retry turned that into a loop. Measured on production 2026-07-28.

Same payload per part as GET /{part_id}/preview-url, keyed by part id. An id the caller cannot see is simply absent from the response rather than 404-ing the batch, so one stale row in a list never blanks the rest.