docs: integrate RDS and ACK architecture memory

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

@@ -2,21 +2,30 @@
 
 ## Current Architecture
 
-{short description of the current system shape}
+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.
 
 ## Main Components
 
 | Component | Responsibility | Notes |
 |---|---|---|
+| Next.js Web | Browser/API routes, domain services, persistence calls, internal Worker tick endpoint | Owns the PostgreSQL pool and `/api/ready`. |
+| Worker | Periodically invokes the internal Worker tick endpoint | No direct database connection or RDS Secret. |
+| PostgreSQL adapter | Backend selection, Pool lifecycle, TLS, parameterized queries, transactions, readiness | Server-only module at `lib/server/database.ts`. |
+| RDS PostgreSQL | Accounts, assets, jobs, usage, templates, billing state | Schema managed by versioned migrations. |
+| Migration Job | Applies migrations and exact application-role privileges | Must complete before Web rollout. |
+| Runtime/object storage | Uploads, generated assets, and logs | Container-local/PVC by default; use OSS/shared storage before scaling Web horizontally. |
 
 ## Important Boundaries
 
-- {boundary that future work should respect}
+- Production backend selection is explicit and fail-closed; never turn a PostgreSQL configuration failure into local JSON fallback.
+- Store callers depend on stable store interfaces, not `pg` or SQL details.
+- Multi-statement consistency uses one transaction client; atomic job claim and wallet posting remain database functions.
+- Database credentials are injected only into Web and migration workloads; Worker uses the internal HTTP boundary.
 
 ## Related Decisions
 
-- {ADR reference}
+- `RDS-001`, `RDS-002`, `ACK-001` in the decision index.
 
 ## Last Updated
 
-{YYYY-MM-DD}
+2026-08-12