Implement Delete Todo endpoint (DELETE /todos/:id)

[landoyjx]

Implement DELETE /todos/:id to remove a Todo item. Response should be 204 No Content on success, or 404 Not Found if the id does not exist.

Acceptance Criteria:

• Returns 204 and deletes the Todo for an existing id
• Returns 404 for a non-existent id
• Subsequent GET /todos/:id for the deleted id returns 404

Test Cases:

• Integration: DELETE /todos/:id with valid id returns 204
• Integration: DELETE /todos/:id with non-existent id returns 404
• Integration: Deleting a todo then calling GET /todos/:id returns 404
• Edge: DELETE /todos/:id with malformed id returns 404 (or 400)