fix(docs): trim parenthetical from postgres-backup.md Last-reviewed line (doc-rot ci-guard)

The doc-rot-detector ci-guard regex is anchored to end-of-line:

  ^>\s*Last reviewed:\s*(\d{4}-\d{2}-\d{2})\s*$

postgres-backup.md had a trailing parenthetical
`(Sprint 4 ACQ — CI restore verification subsection added)` after
the date, which broke the match. Every other doc under docs/ uses
the bare `> Last reviewed: YYYY-MM-DD` form (verified via grep).

The trailing text was historical context that's already captured by
`git log -- docs/operator/runbooks/postgres-backup.md`; doesn't
need to live in the date line.

This guard was masked by the Go Build & Test job aborting at `go mod
tidy` step before the ci-guards step ran — surfacing as a follow-on
failure once that earlier blocker is cleared.

docs/operator/runbooks/postgres-backup.md

@@ -1,6 +1,6 @@
 # Runbook: PostgreSQL backup for certctl
 
-> Last reviewed: 2026-05-16 (Sprint 4 ACQ — CI restore verification subsection added)
+> Last reviewed: 2026-05-16
 
 Use this when:
 - You're setting up a new certctl deployment and need a backup policy