Function push_sender_loop 

async fn push_sender_loop(
    rx: Receiver<PushJob>,
    db: Arc<dyn PushSubscriptionWriter>,
    client: Arc<Client>,
    cache: Arc<DashMap<String, Vec<PushSubscriptionRecord>>>,
    vapid_key: Arc<Vec<u8>>,
)

Single background task that drains the push job queue and sends notifications with bounded concurrency.