fix: create writable data directories in Docker for HF Spaces

HF Spaces runs as non-root user. The backend/data/ directory
(for experiment storage) wasn't in git, so mkdir at runtime
would fail. Pre-create with 777 perms during build. Also fix
.dockerignore to recursively exclude __pycache__.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dockerignore

@@ -2,6 +2,7 @@ node_modules
 frontend/node_modules
 frontend/dist
 .git
-__pycache__
-*.pyc
+**/__pycache__
+**/*.pyc
 .env
+backend/data

Dockerfile

@@ -22,6 +22,10 @@ RUN pip install --no-cache-dir -r backend/requirements.txt
 # Copy backend code
 COPY backend/ ./backend/
 
+# Create writable data directories for runtime (HF Spaces runs as non-root)
+RUN mkdir -p /app/backend/data /app/backend/presets && \
+    chmod -R 777 /app/backend/data /app/backend/presets
+
 # Copy built frontend from build stage
 COPY --from=frontend-build /app/frontend/dist ./frontend/dist/