OpenEMR's Missing Authorization in show-signature.php Allows Portal Patients to Read Staff Signatures

OpenEMR is a free and open source electronic health records and medical practice management application. Versions prior to 8.0.0.3 have a missing authorization check in `portal/sign/lib/show-signature.php` that allows any authenticated patient portal user to retrieve the drawn signature image of any staff member by supplying an arbitrary `user` value in the POST body. The companion write endpoint (`save-signature.php`) was already hardened against this same issue, but the read endpoint was not updated to match. Version 8.0.0.3 patches the issue.