Billing API

GET  /api/v1/billing/plans              # List plans
GET  /api/v1/billing/subscription       # Current subscription
POST /api/v1/billing/checkout           # Create checkout session
POST /api/v1/billing/portal             # Customer portal
GET  /api/v1/billing/invoices           # List invoices
GET  /api/v1/billing/usage              # Usage summary