Implement Bandcamp search service with autocomplete API and HTML
scraping fallback. Add /api/bandcamp/search and /api/bandcamp/embed
endpoints. Create Listening Room page with search, embedded player,
and queue management. Add navigation entry and Bandcamp link on
recommendation cards.
- 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
