Add API documentation for backend endpoints

[kmuwanga83]

📌 Problem

Backend APIs are not well documented, making integration difficult.

🎯 Objective

Provide clear API documentation.

✅ Tasks

• Document all endpoints
• Include request/response examples
• Add authentication details (if applicable)
• Use Swagger or Markdown docs

🧪 Acceptance Criteria

• API documentation is available in /docs or README
• Developers can test endpoints easily
• All endpoints are clearly described