fix: set UV_LINK_MODE=copy in Dockerfile for overlayfs compatibility

uv's reflink-based file cloning fails with EAGAIN inside Docker's
overlayfs. Setting UV_LINK_MODE=copy forces regular file copies.

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Dockerfile

@@ -33,6 +33,9 @@ RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
 # Install uv via pip
 RUN pip install uv
 
+# Disable reflinks — overlayfs doesn't support them (EAGAIN inside Docker builds)
+ENV UV_LINK_MODE=copy
+
 # Copy backend dependency files and install Python dependencies
 COPY backend/pyproject.toml backend/uv.lock /app/backend/
 WORKDIR /app/backend