ci: limit E2E test parallelism to reduce resource contention

Co-Authored-By: Claude <noreply@anthropic.com>

Author: koki-develop

.github/workflows/ci.yml

@@ -90,7 +90,7 @@ jobs:
           echo "Service failed to start"
           exit 1
       - name: Run E2E tests
-        run: go test -v -tags e2e ./e2e/...
+        run: go test -v -tags e2e -parallel 4 ./e2e/...
       - name: Show logs on failure
         if: failure()
         run: docker compose logs