feat(backup-service): Implement MPC backup share storage service
- Add DDD + Hexagonal architecture with NestJS 11.x
- Implement store/retrieve/revoke backup share endpoints
- Add AES-256-GCM double encryption for secure storage
- Add service-to-service JWT authentication
- Add rate limiting (3 retrieves per user per day)
- Add comprehensive audit logging
- Add test suite (37 unit + 21 mock E2E + 20 real DB E2E = 78 tests)
- Add documentation (architecture, API, development, testing, deployment)
- Add Docker and Kubernetes deployment configuration
- Add Prisma 7.x with @prisma/adapter-pg for PostgreSQL
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Co-Authored-By: Claude <noreply@anthropic.com>
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