feat: standardize DeerFlow offline deployment

deploy/linux-docker-offline-deerflow-only/Dockerfile.backend-code-refresh

@@ -0,0 +1,13 @@
+# Fast, deterministic code refresh for a previously built runtime whose
+# dependency manifests are byte-identical to the current source snapshot.
+ARG BASE_IMAGE
+FROM ${BASE_IMAGE}
+
+USER root
+RUN find /app/backend -mindepth 1 -maxdepth 1 -exec rm -rf -- {} +
+COPY . /app/backend
+WORKDIR /app/backend
+
+# Keep the base image's PATH, PYTHONPATH, ENTRYPOINT and CMD. The caller must
+# compare pyproject.toml, uv.lock, requirements-offline.txt and the harness
+# pyproject before using this path.