Design a Highly Scalable URL Shortener System

Design a full system architecture for a globally distributed **URL Shortener** capable of handling billions of redirects per day.

Provide:
1. High-level architecture diagram (text-based)
2. API design (create, fetch, analytics)
3. Database choices (NoSQL vs SQL) with justification
4. Hashing/collision avoidance strategies
5. Read/write optimization strategies
6. Caching layers (Redis/CDN)
7. Data replication strategy
8. Rate limiting + abuse prevention
9. Multi-region failover strategy
10. Capacity planning for 1B requests/day

Finally, provide a refined v2 architecture describing improvements after the system hits scale limits.