Design a Real-Time Chat Messaging Platform

Design a **real-time chat platform** similar to WhatsApp or Messenger.

Include:
1. WebSocket gateway cluster design
2. Presence service architecture
3. Message persistence workflow
4. Fan-out strategies (direct & group)
5. Ordering guarantees (best-effort vs strict)
6. Read receipts + sync logic
7. Offline message delivery pipeline
8. E2E encryption considerations
9. Database sharding strategy
10. Load estimation + scaling plan