fix: Resolve Docker temp directory permission issues

- Updated Dockerfile to create local temp directory with proper permissions
- Added volume mount for backend/temp in docker-compose.yml
- Improved temp directory fallback logic with multiple alternatives
- Added unique timestamps to test files to avoid conflicts
- Better logging to identify which temp directory is being used
- Try process.cwd()/temp before falling back to OS temp directory

This should resolve the persistent EACCES permission errors in Docker
containers by ensuring the backend uses a writable temp directory.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

docker/Dockerfile.backend

@@ -30,10 +30,11 @@ RUN addgroup -g 1001 -S nodejs && \
     adduser -S nodejs -u 1001
 
 # Create necessary directories with proper permissions
-RUN mkdir -p logs /tmp/minio-webui-temp && \
+RUN mkdir -p logs temp /tmp/minio-webui-temp && \
     chown -R nodejs:nodejs /app && \
     chown nodejs:nodejs /tmp/minio-webui-temp && \
-    chmod 755 logs /tmp/minio-webui-temp
+    chmod 755 logs temp /tmp/minio-webui-temp && \
+    chmod 755 /app/temp
 
 # Switch to non-root user
 USER nodejs