Prompt

Design push notification system.

<notification_types>
{transactional, marketing, etc}
</notification_types>

<platforms>
{iOS, Android, web}
</platforms>

<scale_requirements>
{number of users and frequency}
</scale_requirements>

Design:
1. Provider selection
2. Token management
3. Segmentation strategy
4. Delivery optimization
5. Analytics tracking
6. Opt-out handling