- Bandcamp: match artist+song, fall back to artist-only page
- YouTube fallback: if not on Bandcamp, link to YouTube music video search
- Fix My Playlist: AI analyzes playlist, finds outliers, suggests replacements
- User chooses recommendation count (5, 10, 15, 20)
- 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