Add HandleGenerateVAPIDKeys (POST /admin/push/generate-vapid-keys) and HandleSendTestPush (POST /admin/push/test) to PushHandler, with a new pushNotifier interface and nil-tolerant third param on NewPushHandler. Update both router.go call sites to pass deps.NotificationsService. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>