Replace COPY README.md with RUN echo stub to avoid .dockerignore exclusion. The *.md glob in .dockerignore blocked the COPY; a generated stub satisfies hatchling metadata validation without any .dockerignore changes.
Stage README.md at /README.md during backend image dependency sync so hatchling metadata validation for ../README.md succeeds in deployable-image purity build step.
Add Dockerfile boundary checks and deployable image purity validation for backend/frontend runtime artifacts. Wire enforcement into CI workflows and document runtime-vs-validation ownership.