Design a Recommendation Engine Backend

Create a backend architecture for a **recommendation engine** (content-based + collaborative filtering).

Include:
1. Feature pipeline
2. Real-time vs batch inference models
3. Offline training architecture
4. Ranking & scoring layer
5. Embedding store
6. A/B testing framework
7. Personalized feed generation
8. Cold start strategies
9. Feedback loops
10. Monitoring model drift