Track push notification open

Track when a user opens a push notification. **Request Body:** - **pushNotificationId**: ID of the push notification being opened - **deviceToken**: Device token (optional, for additional tracking) - **userId**: User ID (optional, for additional tracking) - **eventData**: Additional event data (optional) - **occurredAt**: When the open event occurred (optional, defaults to now) **Use Cases:** - Track notification engagement - Analytics and reporting - User behavior analysis - Notification optimization