Fix url set to null

[GianniCarlo]

No description provided.

[github-actions]

✅ Claude PR Review — PASS

This PR changes a single line in LibraryService.parseLibraryItemDb, setting the API-output url field to null instead of an empty string to fix an iOS client decoding bug (URL? fails to decode '', wedging the upload retry loop). The LibraryItem.url type is already string | null | undefined, so the change is type-safe. There are no authorization, auth-middleware, IAP/passkey, SQL, or migration concerns in this diff — it only affects the shape of a client-facing response field. No issues found.

Findings: no findings

_{Model claude-opus-4-8 · run log · 0 new · 0 carried over · 0 resolved · advisory (a human should still review). Duplicate findings are de-duplicated and stale ones auto-resolved across pushes.}