Enhance notification system with in-app delivery support

- Added `DeliveryChannelInApp` to support in-app notifications.
- Introduced `EventTypeInApp` constant for identifying in-app notifications.
- Updated `AdminNotifications` and `CustomerNotifications` handlers to set the event type to `EventTypeInApp`.
- Modified the `GetByUserID` repository method to handle filtering for in-app notifications, allowing for more flexible retrieval options.
- Updated the `persistNotification` method to include in-app as a delivery method.

This update improves the notification system by enabling in-app notifications, enhancing user engagement and notification management capabilities.

internal/notification/entity.go

@@ -25,8 +25,12 @@ type DeliveryChannel string
 const (
 	DeliveryChannelPush  DeliveryChannel = "push"
 	DeliveryChannelEmail DeliveryChannel = "email"
+	DeliveryChannelInApp DeliveryChannel = "in_app"
 )
 
+// EventTypeInApp identifies notifications stored for the in-app notification center.
+const EventTypeInApp = "in_app"
+
 // DeliveryStatus represents the delivery status of notification
 type NotificationStatus string