Fixes the boot-storm half of issue #29905: kanban_notify_subs.last_event_id
defaulted to 0, so a subscription created on an already-active task replayed
the task's ENTIRE terminal-event backlog on the next notifier tick. With
many stale subs (27 observed in the report) a gateway boot after downtime
burst 100+ notifications in one go.
add_notify_sub now snaps the cursor to COALESCE(MAX(task_events.id), 0) for
the task inside the same INSERT, so new subscriptions start caught up and
only receive events that occur AFTER subscribing. The gateway slash-command
and kanban-tool auto-subscribe paths run at task creation, where the
snapshot is just the 'created' event — behavior there is unchanged.
Stale fixtures that asserted the literal 0 creation cursor now assert
'cursor unchanged/unclaimed' instead, which is what they actually meant.
