feat(users): implement user disabling functionality with admin controls
- Add `disabled` column to users table via migration
- Update login flow to reject disabled accounts with 403 error
- Introduce new `/admin/users` route for admin-only user management (disable/enable, verify email, delete with content)
- Extend users service to support listing all users, toggling disabled status, admin verification, and cascading deletion of user's songs/playlists
- Add admin UI for managing users with visual indication of disabled accounts (opacity reduction)
- Update navigation to include "Users" link for admins
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Brian Fertig