fix(ci): stub readme for hatchling rather than COPY

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.

Dockerfile.backend

@@ -19,8 +19,10 @@ ENV PATH="$VIRTUAL_ENV/bin:$PATH"
 
 # Copy dependency files first for better caching
 COPY backend/pyproject.toml backend/uv.lock* ./
-# Hatchling metadata references ../README.md from backend/pyproject.toml.
-COPY README.md /README.md
+# Hatchling resolves the readme path ../README.md relative to the backend
+# package root (/app). Create a stub so metadata validation succeeds without
+# requiring the file to pass through .dockerignore.
+RUN echo '# plex-playlist' > /README.md
 
 # Install dependencies
 RUN uv sync --frozen