docs: integrate RDS and ACK architecture memory

.project-docs/20-architecture/data-flow.md

@@ -4,15 +4,23 @@
 
 | Flow | Source | Destination | Notes |
 |---|---|---|---|
+| Web persistence | Routes/services/stores | PostgreSQL adapter -> RDS | Parameterized SQL; related statements share one Pool client transaction. |
+| Worker processing | Worker process | Internal Web Service `/api/internal/worker/tick` | Authenticated by internal token; Worker has no RDS credentials. |
+| Schema rollout | ACK migration Job | RDS PostgreSQL | Versioned checksummed migrations under an advisory lock. |
+| Readiness | ACK probe | Web `/api/ready` -> RDS | Verifies connection, 11 runtime tables, required privileges, and 2 functions. |
 
 ## State Ownership
 
-- {state owner or persistence rule}
+- Production relational state belongs to RDS PostgreSQL when `ZHINIAN_DATA_BACKEND=postgres`.
+- Local JSON under the runtime directory is an explicit development/test backend, not a production fallback.
+- Uploads/generated files remain runtime/object-storage state and are not made shared by the PostgreSQL migration.
 
 ## External Interfaces
 
-- {API, file, service, or user-facing boundary}
+- Alibaba Cloud RDS PostgreSQL via its internal endpoint and verified TLS CA.
+- Alibaba Cloud ACK resources under `deploy/ack/`.
+- Internal Worker HTTP endpoint is cluster-internal and blocked from public Ingress routing.
 
 ## Last Updated
 
-{YYYY-MM-DD}
+2026-08-12