Phase 6: Production hardening — Postgres, KMS, scaling, testing

[kitplummer]

From ADR-055 Phase 6.

• Postgres backend with envelope encryption for key material
• KMS integration (AWS KMS, HashiCorp Vault) for root key protection
• Horizontal scaling with leader election per formation → ADR: Horizontal scaling strategy #21 (ADR: defer implementation, PR ADR: Horizontal scaling strategy #46)
• Formation sharding across gateway replicas → Formation sharding across gateway replicas #22 (folded into ADR: Horizontal scaling strategy #21)
• Load testing: concurrent formations, CDC throughput, enrollment rate → Load testing: concurrent formations, CDC throughput, enrollment rate #23 (PR Add load-test CLI subcommand #25)
• Integration test suite: multi-org isolation, CDC end-to-end, OIDC flow → Org data isolation verification #24
• Org data isolation verification (no cross-tenant leaks) → Org data isolation verification #24