amal66 puts an expiry date on document download links

The change would give signed document links a 30-day lifespan. Once expired, a copied link from an old export, forwarded email or system log would no longer open the underlying file.

Just as importantly, it takes a fail-closed stance on older links. Previously issued links without an expiry would be rejected, rather than grandfathered in indefinitely. The proposal also strengthens the check used to confirm that a link has not been tampered with.

This was a security hardening proposal, not a shipped change: the pull request closed without merging.