Fix notebook directory permissions for non-root user

Pre-create /app/notebooks/{user,examples} with correct ownership
before switching to the spicebook user. Without this, the app crashes
with PermissionError when it tries to create these directories at
runtime.

backend/Dockerfile

@@ -46,7 +46,9 @@ RUN --mount=type=cache,target=/root/.cache/uv \
     uv pip install --system .
 
 # Run as non-root
-RUN useradd --create-home --shell /bin/bash spicebook
+RUN useradd --create-home --shell /bin/bash spicebook && \
+    mkdir -p /app/notebooks/user /app/notebooks/examples && \
+    chown -R spicebook:spicebook /app/notebooks
 USER spicebook
 
 EXPOSE 8000