Add ff infrastructure: shared namespaces, CNPG operator, Postgres clusters

[rgardner4012]

Summary

• Introduces infra/shared-namespaces/ — single owner of namespaces shared across multiple Flux Kustomizations. ff-dev and ff-production are the first consumers (see ADR 008)
• Adds infra/databases/cloudnativepg/ — CNPG operator via Helm into cnpg-system
• Adds infra/databases/postgres/ — separate ff-postgres CNPG Cluster per environment:
  • dev: 1 instance, PostgreSQL 16, 5Gi (disposable)
  • production: 3 instances (1 primary + 2 standbys), PostgreSQL 16, 10Gi

Dependency order

infra-shared-namespaces
        ↓
infra-cloudnativepg  (wait: true)
        ↓
infra-ff-postgres

Docs updated

• docs/adr.md — ADR 008: shared-namespaces pattern
• docs/disaster-recovery.md — CNPG backup checklist items, RTO/RPO table entries, ff-postgres recovery runbook

Notes

• CNPG auto-creates ff-postgres-app Secret in each namespace with a uri key — consumed by the ff app as DATABASE_URL
• CNPG backup target not yet configured — tracked in disaster recovery checklist