Force a one-shot run of the pending-delete purger

T-124 — Calls PendingDeletePurger.runOnce() directly and returns the count of accounts purged. Useful when an account has just passed its grace window and the operator doesn’t want to wait for the next scheduled sweep (default cadence: 1 hour). Idempotent: re-running with no eligible rows returns 0. Only sweeps accounts already past pending_deletion_at + grace_days — does not bypass the grace window. Emits an admin.account_deletion.purge_now audit row.