Add endpoint to allow users to set security questions during registration

Description: We need an endpoint that allows setting security questions and answers for a specific user (by user ID). This will help support account recovery and additional authentication flows in the future.

Endpoint:

• Method: POST
• Path: /users/{user_id}/security-questions

Example Request Body:

{
  "questions": [
    {
      "question": "What was the name of your first pet?",
      "answer": "Fluffy"
    },
    {
      "question": "What is your mother’s maiden name?",
      "answer": "Smith"
    }
  ]
}

Requirements

• Validate that the user_id corresponds to an existing user.
• Accept a slice of 2–3 questions and answers.
• Hash the answers before saving (do not store plain text answers).
• Store them in the DB.
• Return 200 OK on success.
• Return 400 Bad Request for validation errors.
• Return 404 Not Found when the user doesn't exist with the given ID.
• Return 500 Internal Server Error when appropriate.
• Include basic validation: ensure both question and answer are non-empty strings.