Create a new GET /courses_user endpoint

[kyleecodes]

Overview

To optimize user data loading, introduce a new GET /courses_user endpoint that retrieves session and course data specifically for courses or session pages. This means we can remove courses and user data from the profile.

This issue is not blocked, but ideally #717 should be completed first.

Action Items

• Create a new GET /courses_user endpoint in the backend.
• Implement the getCourseUserByUserId service method for this endpoint.
• Write backend tests to ensure proper functionality (alternatively, this can be made into a separate issue.)
• Add a new controller file for handling this endpoint.
• Include code comments.