UX: Custom downloaded PDF filenames

[ptabor]

Summary

Per user request, this PR sets the downloaded PDF filename based on the song/songbook title and the current date (e.g., Spiewnik_2026-06-12.pdf).

1. Added an optional filename query parameter to the backend /api/jobs/{id}/download endpoint which sets the Content-Disposition header.
2. Updated the frontend (both song editor and songbook editor) to append the generated filename when displaying/opening the download link.