Stage 0: scaffold monorepo, backend skeleton, docs, CI

- go.work (Go 1.26.3) with backend module; deps added incrementally (gin+zap only)

- backend: /healthz + /readyz, env config, graceful shutdown

- docs: ARCHITECTURE, FUNCTIONAL (+ru mirror), TESTING

- PLAN.md (stage tracker + per-stage open details) and CLAUDE.md (per-stage workflow)

- .gitea go-unit CI (gofmt/vet/build/test)

.gitea/workflows/go-unit.yaml

@@ -0,0 +1,56 @@
+name: Tests · Go
+
+# Fast unit tests for the Go side of the monorepo. Runs on every push and pull
+# request whose path filter matches a Go source directory. The module list
+# grows as new go.work modules (gateway, pkg/*, platform/*) are added by later
+# stages.
+
+on:
+  push:
+    paths:
+      - 'backend/**'
+      - 'go.work'
+      - 'go.work.sum'
+      - '.gitea/workflows/go-unit.yaml'
+      - '!**/*.md'
+  pull_request:
+    paths:
+      - 'backend/**'
+      - 'go.work'
+      - 'go.work.sum'
+      - '.gitea/workflows/go-unit.yaml'
+      - '!**/*.md'
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.work
+          cache: true
+
+      - name: gofmt
+        run: |
+          unformatted="$(gofmt -l .)"
+          if [ -n "$unformatted" ]; then
+            echo "gofmt needed on:"; echo "$unformatted"; exit 1
+          fi
+
+      - name: vet
+        run: go vet ./backend/...
+
+      - name: build
+        run: go build ./backend/...
+
+      - name: test
+        # -count=1 disables the test cache so a green run never depends on a
+        # previous runner's cached state.
+        run: go test -count=1 ./backend/...