Implement Create Todo endpoint (POST /todos)

[landoyjx]

Implement POST /todos to create a new Todo item. Request body must contain title (string, non-empty). Response should be 201 Created with the full Todo object.

Acceptance Criteria:

• Returns 201 and the created Todo on success
• Returns 400 if title is missing, null, or empty string
• createdAt and updatedAt are set automatically

Test Cases:

• Integration: POST /todos with { "title": "Buy milk" } returns 201 and a Todo object with id, title, completed=false, createdAt, updatedAt
• Integration: POST /todos with { "title": "" } returns 400 with JSON error
• Integration: POST /todos with {} returns 400 with JSON error
• Edge: POST /todos with extra fields ignores them and still creates the Todo