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
