Invoice generation and retrieval is implemented

Users who cannot retrieve their own invoices open support tickets for every tax filing, expense report, and accounting reconciliation, burning engineering and support cycles on a solved problem. For B2B customers, missing invoice access blocks procurement workflows — finance teams will not pay invoices they cannot download as PDFs with proper tax metadata. This maps directly to user-experience taxons and creates churn risk when customers cannot close their books at month-end.

Add a Stripe Customer Portal session endpoint and link it from the account settings page — the portal handles invoice listing, PDF downloads, and receipts with zero custom UI. Create app/api/billing/portal/route.ts that calls stripe.billingPortal.sessions.create({ customer, return_url }) and redirect the user to portalSession.url.