docs: Add comprehensive implementation guides and API specs

- Add ENHANCEMENT_DELIVERY_SUMMARY.md mapping all 9 user requests to sections
- Add IMPLEMENTATION_GUIDE.md with phase-by-phase development plan
- Add QUICK_REFERENCE.md with quick navigation and examples
- Add doc/api/persistence_api.md with complete REST API specification
- Includes deployment guides, testing strategies, metrics, and use cases

Author: vinod0m