Implement robust environment variable configuration for different deployment environments.

📋 Tasks

• Create configuration for development, testing, and production environments
• Implement environment variable validation
• Document required environment variables
• Create example configuration files

🔧 Technical Details

• Use dotenv v16.4.5 as specified in package.json
• Required environment variables:
  • PORT: For Express server
  • MONGODB_URI: MongoDB connection string
  • JWT_SECRET: Secret for signing JWT tokens
  • JWT_EXPIRES_IN: Token expiration time
  • NODE_ENV: Environment (development/production)
  • EMAIL_SERVICE: For email notifications (if implementing)
  • EMAIL_USER: For email authentication
  • EMAIL_PASS: For email authentication
• Implement validation to ensure all required variables are present
• Provide fallback values where appropriate

✅ Acceptance Criteria

• Environment configuration works for all environments
• Missing required variables trigger appropriate errors
• Documentation includes all required variables
• Example configuration files are provided
• Environment variables support all application needs