Implement todo API

[jazferlex]

This PR implements a simple REST API for managing todos using Express.

Features:

• GET /api/todos - Retrieve all todos
• GET /api/todos/:id - Retrieve a single todo by ID
• POST /api/todos - Create a new todo
• PUT /api/todos/:id - Update an existing todo
• DELETE /api/todos/:id - Delete a todo

Details:

• Data is stored in memory (no database)
• Each todo includes: id, title, completed, createdAt
• Basic validation and error handling implemented
• Proper HTTP status codes returned (400, 404, 201, 204)

Tested using curl to verify all endpoints and edge cases.