Add architecture overview page

Architecture.md

@@ -0,0 +1,30 @@
+# Architecture Overview
+
+## Network topology
+
+```
+Internet → Route 53 → ALB (public) → EKS nodes (private) → RDS (isolated)
+                                   ↘ ElastiCache (private)
+```
+
+## Services map
+
+| Service | Technology | Owner | Namespace |
+|---------|-----------|-------|-----------|
+| API | Rust/Axum | backend team | `nexus` |
+| Frontend | React/Vite (static CDN) | frontend team | — |
+| Data pipeline | Python/Airflow | data team | `airflow` |
+| Auth | OIDC (Keycloak) | platform team | `auth` |
+| Monitoring | Prometheus + Grafana | platform team | `monitoring` |
+
+## Deployment flow
+
+```
+PR merged to main
+  → GitHub Actions CI
+  → Docker build + push to registry.nexus.local
+  → Helm upgrade (nexus-staging) — auto
+  → Smoke tests
+  → Manual approval gate
+  → Helm upgrade (nexus-prod)
+```