docs: integrate approved Next Go architecture

.project-docs/20-architecture/system-overview.md

@@ -4,6 +4,22 @@
 
 The Next.js application runs as a Web workload with a separate HTTP-polling Worker. Production server state is stored directly in PostgreSQL through a shared server-only adapter; development and tests can explicitly use local JSON. ACK deploys database migration, Web, Worker, Service, and Ingress resources separately.
 
+This remains the implemented and deployable architecture. No Go backend code or Go deployment resources have been integrated.
+
+## Approved Target Architecture
+
+The accepted target in ADR-003 is a same-origin Next.js frontend plus Go modular-monolith backend:
+
+| Target component | Responsibility | Constraint |
+|---|---|---|
+| Next.js frontend | Pages, static assets, SSR, browser UI | Calls Go over HTTP; no RDS/provider/OSS/business Secret. |
+| Go backend | Existing HTTP/file contracts, identity, administration, assets, jobs, billing, usage, providers, storage, Webhooks, readiness | Owns relational access and initially embeds WorkerLoop. |
+| RDS PostgreSQL | Relational state and cross-instance concurrency | Retains versioned migrations and both concurrency-sensitive database functions. |
+| Migration Job | Schema and application-role grants | Remains one-shot and separate from long-lived workloads. |
+| Alibaba Cloud OSS | Shared generated/uploaded assets | Must be production-ready before horizontal workload scaling. |
+
+Ingress will eventually route page/static paths to Next.js and `/api`, `/uploads`, and `/generated-results` to Go. The initial target remains two long-lived Pods (`Next x1 + Go x1`). This target is not yet implemented.
+
 ## Main Components
 
 | Component | Responsibility | Notes |
@@ -24,7 +40,8 @@ The Next.js application runs as a Web workload with a separate HTTP-polling Work
 
 ## Related Decisions
 
-- `RDS-001`, `RDS-002`, `ACK-001` in the decision index.
+- Current implementation: `RDS-001`, `RDS-002`, and `ACK-001`.
+- Accepted target: `ADR-003`; it supersedes ACK-001 only after verified implementation and cutover.
 
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