Allow non-team GitHub users to access public courses

[MioYuuIH]

Summary

We want to support a softer access model for GitHub-authenticated users.

Today, access is effectively tied to team membership. We want GitHub users who are not in the configured team to still be able to log in and access a limited subset of courses marked as public, while keeping protected/private courses restricted to authorized users.

This requires separating authentication from course visibility/authorization more clearly.

Proposed scope

• Allow GitHub-authenticated users outside the configured team to sign in
• Introduce a way to mark selected courses/resources as public
• Ensure non-team users only see or can select public courses
• Preserve current behavior for team-authorized users
• Preserve current restrictions for non-public courses

Acceptance criteria

• A GitHub-authenticated user outside the team can complete login
• Non-team users can only view/select courses marked as public
• Private courses are not exposed to non-team users
• Team member behavior remains unchanged
• Public/private course visibility is configurable rather than hardcoded