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-compose.yml

@@ -15,6 +15,7 @@ services:
       - "${PORT:-7510}:${PORT:-7510}"
     volumes:
       - ./logs:/app/logs
+      - ./backend/temp:/app/temp:rw
       - ~/.mc:/home/nodejs/.mc:ro
     depends_on:
       - redis