Implement worker thread for notification service

[sulaiman2909]

Worker Thread Implmentation for Notification Service

• Created the NotificationWorker class to process notifications asynchronously.
• Used a LinkedBlockingQueue to queue incoming notification tasks.
• Set up a ThreadPoolExecutor to handle notifications concurrently.
• Added logic to ensure that worker thread is initialized only once to minimize unnecessary runtime of the notification service from the moment of login until a notification is triggered.