Fix Python path issue - add PYTHONPATH for app module

Dockerfile

@@ -20,6 +20,9 @@ COPY backend /app/backend
 # Copy frontend build into /app/frontend/dist
 COPY --from=frontend-build /frontend/dist /app/frontend/dist
 
+# Set PYTHONPATH so Python can find the app module
+ENV PYTHONPATH=/app/backend:$PYTHONPATH
+
 # HF Spaces uses port 7860
 EXPOSE 7860