Design a notification system that receives alert events from other internal systems and notifies all subscribed users through multiple channels (e.g., in-app/push, email, SMS). Your design should address: - Core requirements and APIs (publish alert, manage subscriptions/preferences, send notifications) - Fanout to many subscribers - Multiple delivery channels with different providers - Reliability (retries, dead-letter queues), ordering/deduplication, and idempotency - Rate limiting and user preferences (opt-out, quiet hours) - Observability and operational concerns Assume high volume (millions of alerts/day) and that different alert types may have different urgency and SLAs.