Extend NotificationsRetentionTask with an optional PushPurger dependency
that deletes terminal push_deliveries rows older than 7 days. Defines the
PushPurger interface in the tasks package; *push.Store satisfies it.
main.go call site updated to pass nil (real store wired in Task 15).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>