Enhance notification entity and service for improved delivery channel handling

- Introduced new event types for notifications, including email and push, to support a broader range of delivery methods.
- Added a `persistedEventType` function to map delivery channels to their corresponding event types, improving clarity and maintainability.
- Updated the `sendToRecipient` method to ensure in-app notifications are always persisted, enhancing the reliability of notification records.
- Improved error logging for in-app notification persistence failures, providing better context for debugging.

This update enhances the notification system's flexibility and reliability by supporting multiple delivery channels and improving error handling.

internal/notification/entity.go

@@ -28,8 +28,26 @@ const (
 	DeliveryChannelInApp DeliveryChannel = "in_app"
 )
 
-// EventTypeInApp identifies notifications stored for the in-app notification center.
-const EventTypeInApp = "in_app"
+// Persisted event types for stored notifications (admin list / notification center).
+const (
+	EventTypeInApp  = "in_app"
+	EventTypeEmail  = "email"
+	EventTypePush   = "push"
+)
+
+// persistedEventType maps a delivery channel to the event_type stored in MongoDB.
+func persistedEventType(channel DeliveryChannel) (string, bool) {
+	switch channel {
+	case DeliveryChannelEmail:
+		return EventTypeEmail, true
+	case DeliveryChannelPush:
+		return EventTypePush, true
+	case DeliveryChannelInApp:
+		return EventTypeInApp, true
+	default:
+		return "", false
+	}
+}
 
 // DeliveryStatus represents the delivery status of notification
 type NotificationStatus string