Refactor the xtream-proxy from a single-file Express-based server to a
modular Node.js application using native http/https modules. This
change introduces a major version bump to 2.0.0.
- Replace `hosts.txt` with `upstreams.json` for configuration
- Remove heavy dependencies (`express`, `ws`) in favor of native modules
- Implement new environment variables for proxy credentials and timeouts
- Add `src/` directory for modular code structure
- Add `docker-compose.example.yml` for easier deployment
- Update Dockerfile to support the new directory structure and environment
- Improve error handling and probe logic for upstream selection