ci: enforce gofmt on the Go backend

The package is fully gofmt-clean after the modernization pass, so the
formatting check deferred from the initial CI setup can now be enabled.

.github/workflows/ci.yml

@@ -55,6 +55,9 @@ jobs:
           go-version: "1.25.8"
           cache-dependency-path: go_backend/go.sum
 
+      - name: Check formatting
+        run: test -z "$(gofmt -l .)"
+
       - name: Vet
         run: go vet ./...