feat: add Go migration compatibility foundation

backend/README.md

@@ -0,0 +1,34 @@
+# Go backend foundation
+
+This directory contains the first implementation slice of ADR-003. It is a
+runnable compatibility foundation, but it is **not** the current production API
+owner: Next.js, the Node Worker, Docker Compose, and the ACK manifests remain
+unchanged until later route-by-route cutover work passes the shared contracts.
+
+Implemented Modules:
+
+- `identity`: legacy `zhinian_session` HMAC, parsing, and chunking contract.
+- `postgres`: fail-closed configuration, verified-CA TLS, readiness, and calls
+  to the existing atomic claim and wallet PostgreSQL functions.
+- `httpapi`: process health and database readiness handlers.
+- `application`: composition and the `cmd/zhinian-api` process entry point.
+
+From the repository root:
+
+```bash
+npm run go:fmt
+npm run go:test
+npm run go:vet
+npm run go:build
+```
+
+The runner defaults to `CGO_ENABLED=0` for reproducible cross-platform builds.
+To exercise the local foundation manually without changing the existing Next
+server, use a different port:
+
+```bash
+ZHINIAN_DATA_BACKEND=local GO_BACKEND_PORT=8080 ./backend/zhinian-api
+```
+
+Only `/api/health` and `/api/ready` are implemented in this foundation. No
+Ingress, Docker, ACK, Secret, or Worker ownership has moved to Go yet.