purge_stale_done_notify_subs only matched status='done', so a task the
circuit breaker parked in 'blocked' kept its notify-sub rows forever on
boards that never archive. Widen the predicate to done OR blocked while
keeping the existing age clause; backlog/ready cards are idle, not
abandoned, and stay exempt (test_gc_spares_reopened_task_even_when_old).
Watcher comment/log and docs updated to say done/blocked.
Closes#100955
Co-authored-by: itsflownium <itsflownium@users.noreply.github.com>