Google Workspace Event Integration

[gentij]

This PR implements the backend challenge requirements with the following features:

• REST API endpoints for sources management
• Retry mechanism for failed jobs
• Scheduling system for periodic data fetching
• Error handling and validation
• Docker containerization

The solution is organized in the backend/ folder with complete setup instructions in the README.md.

Tech stack:

• Node.js/Express
• MongoDB
• Redis/BullMQ