Add user authentication and role-based access control

[mflorezr-git]

Implement user authentication (e.g., OAuth2, OpenID Connect, or simple username/password) and support for roles (student, parent, admin, provider). Restrict endpoints based on user roles and permissions. This will enable secure sign-up, activity management, and admin workflows.

[mflorezr-git]

User authentication and role-based access control (RBAC) have been implemented using OAuth2 and JWT, restricting activity sign-up and unregister endpoints to authenticated students. Thank you all for your ideas and feedback!