CLSA-2026-1779697425 postgresql: Fix of CVE-2026-6478

CVE-2026-6478: backport upstream prerequisite that introduces the timingsafebcmp constant-time memory comparison helper, then apply it to SCRAM and MD5 authentication paths that previously used memcmp or strcmp on password hashes, computed keys, and SCRAM nonces, to prevent timing-side-channel...