PdfDing: Shared PDF Expiration, Max Views, and Deletion Bypass via Serve/Download Endpoints

PdfDing is a selfhosted PDF manager, viewer and editor offering a seamless user experience on multiple devices. Prior to version 1.7.1, check_shared_access_allowed() validates only session existence — it does not check SharedPdf.inactive (expiration / max views) or SharedPdf.deleted. The Serve and Download endpoints rely solely on this function, allowing previously-authorized users to access shared PDF content after expiration, view limit, or soft-deletion. This issue has been patched in version 1.7.1.