Update README with architecture details

Added architecture image and description for scaling model.

Author: Ganesh-a0576

README.md

@@ -34,6 +34,7 @@ Key engineering decisions made:
 ---
 
 ##  Architecture
+<img width="2437" height="678" alt="null (2)" src="https://github.com/user-attachments/assets/f7c84c11-0cf1-4a3e-b21c-fc0eb12a0bff" />
 
 
 **Scaling model:** Multiple API instances share a single Redis cache. Rate counters are centralized — adding instances does not inflate per-user limits.