Implement waitlist notification when resources are returned

As a librarian, when a resource is returned, the system should automatically retrieve users who are waiting for that resource and send them a notification email informing them that the resource is now available for borrowing. This feature is intended to enhance the user experience and ensure that users are notified when a resource is available.

• Design and implement a database query to retrieve users waiting for resources from a waiting list.
• Implement the ability to send emails to ensure that notifications can be sent to all users waiting for this resource.
• Ensure that the system is able to retrieve the waiting user and send a notification as soon as the resource has been successfully returned.