Library item card: two minor follow-ups from #2097 review

[jaylfc]

Two MINOR findings from CodeRabbit on #2097 (merged clean; these are non-blocking polish):

1. Broken thumbnail fails silently (LibraryItemCard.tsx). When a thumbnail artifact exists but the image fails to load, onError hides the <img>, leaving blank space. Prefer showing the same placeholder used when no thumbnail exists.
2. source option declared but never sent (lib/library.ts). IngestLibraryOptions.source is defined but not serialized into the request body. Either wire it into the POST body or drop the option.

Good first task for a fleet lane.