Adds new Authentik (v2025.10.0) authentication/SSO stack with:
- Redis cache service
- Authentik server (exposed at auth.frostlabs.me via Traefik)
- Authentik worker for background tasks
- Fixed depends_on references to use correct service name (redis)
- External PostgreSQL backend at 10.0.4.10
- Docker secrets integration for sensitive credentials
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
