The authorized-user backdated-entry guard used MariaDB-only
`MAX(timestamp(posting_date, posting_time))`, invalid on Postgres. Convert to
`Max(posting_datetime)` (the precomputed column) via frappe.qb. Same result on
MariaDB; now valid under Postgres.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>