* feat: add invoice URL to proration and test seed scripts - Add invoiceUrl field to MonthlyProration schema - Update billing service to return hosted_invoice_url after finalizing - Save invoice URL when creating proration invoices - Add seed script for testing proration with real Stripe data - Add cleanup script for test data removal Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> * add invoice url * add invoice URL to tests --------- Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>