- Add stripe_customer_id and stripe_subscription_id fields to User model
- Add Stripe config settings (secret key, publishable key, price ID, webhook secret)
- Create billing API endpoints: checkout session, webhook handler, portal, status
- Add frontend Billing page with upgrade/manage subscription UI
- Add billing route and Pro nav link
- Add stripe dependency to requirements
- Add ytmusicapi dependency for fetching public YouTube Music playlists
- Create youtube_music service with playlist fetching and track search
- Add /api/youtube-music/import and /api/youtube-music/search endpoints
- Add importYouTubePlaylist and searchYouTubeMusic API client functions
- Update Playlists page with YouTube Music import button and URL input modal