Add Pushover Notification Support

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This PR adds comprehensive Pushover notification support to the Node Hello application.

🚀 Features Added

Dual Mode Operation

• HTTP Server Mode (default): Continues to serve 'Hello Node!' as before
• Pushover Mode: Send notification and exit immediately

Command Line Interface

• --pushping: Enable pushover notification mode
• --userkey: Pushover user key (required with --pushping)
• --apikey: Pushover API key (required with --pushping)
• --title: Custom notification title (optional)
• --message: Custom notification message (optional)
• --port: Custom server port (for HTTP mode)
• --help: Show comprehensive help

Enhanced Package Scripts

• npm run notify: Quick notification sending
• npm run help: Show help information

📋 Usage Examples

HTTP Server Mode (unchanged)

npm start                    # Default port 3000
node index.js --port 8080    # Custom port

Pushover Mode (new)

# Send notification with default message
node index.js --pushping --userkey YOUR_USER_KEY --apikey YOUR_API_KEY

# Send custom notification
node index.js --pushping --userkey YOUR_USER_KEY --apikey YOUR_API_KEY --title "Alert" --message "Task completed"

📚 Documentation

• Updated README.md with comprehensive usage guide
• Enhanced API_DOCUMENTATION.md with Pushover integration details
• Added command line help system

🔧 Technical Details

• Zero additional dependencies (uses Node.js built-in modules)
• Backward compatible - existing usage unchanged
• Proper error handling and validation
• Graceful shutdown handling
• Comprehensive parameter validation

✅ Testing

• HTTP server mode works as before
• Pushover notifications send successfully
• Command line parsing handles all parameters correctly
• Error messages are clear and helpful

This maintains full backward compatibility while adding powerful notification capabilities.