fix(ci): include root readme for backend uv sync

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.

Dockerfile.backend

@@ -19,6 +19,8 @@ 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
 
 # Install dependencies
 RUN uv sync --frozen