Add Stripe subscription billing integration

- 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

frontend/src/components/Layout.tsx

@@ -1,6 +1,6 @@
 import { useState } from 'react'
 import { Link, useLocation, useNavigate } from 'react-router-dom'
-import { Disc3, LayoutDashboard, ListMusic, Compass, Heart, Menu, X, LogOut, User } from 'lucide-react'
+import { Disc3, LayoutDashboard, ListMusic, Compass, Heart, Crown, Menu, X, LogOut, User } from 'lucide-react'
 import { useAuth } from '../lib/auth'
 
 const navItems = [
@@ -8,6 +8,7 @@ const navItems = [
   { path: '/playlists', label: 'Playlists', icon: ListMusic },
   { path: '/discover', label: 'Discover', icon: Compass },
   { path: '/saved', label: 'Saved', icon: Heart },
+  { path: '/billing', label: 'Pro', icon: Crown },
 ]
 
 export default function Layout({ children }: { children: React.ReactNode }) {