feat: Initialize Node.js backend foundation

This commit lays the groundwork for the HR Platform's backend.

Key changes include:
- Initialized a Node.js project with `npm`.
- Added `express` as the core web framework.
- Established the backend source code structure in the `src/` directory.
- Implemented a basic Express server in `src/index.js`.
- Added a `/health` endpoint for monitoring.
- Included a centralized error-handling middleware.
- Configured the server port to be read from an environment variable (`process.env.PORT`).
- Added a `start` script to `package.json` for running the application.
- Created a log file for Agent 2 to document backend development progress.

.gitignore

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+node_modules