Role-based gating: free users see premium webinars but can't watch

**Sprint 6 - Webinars & Content Gating**

Implement role-based content gating for webinars:
- Free users can see premium webinars listed on the navigation page
- Free users cannot access the video of premium webinars
- Paid subscribers have full access to all webinar content
- There are webinars for all users and webinars only for paid subscribers