Retention purge deletes Forwarded/Reconciled rows older than the cutoff and
reclaims pages via incremental_vacuum; Pending rows are never purged on age
alone (would lose audit data that never reached central). Temp-table purge set
avoids the SQLite 999-param limit; runs under _writeLock. (PLAN-04 Task 1, S1/U2)
Joseph Doherty