Stripe SDK was being initialized at module load time, which failed
during Next.js build when STRIPE_SECRET_KEY wasn't set in the build
environment. Changed to lazy initialization pattern with proxy object
that creates the client on first access.
Fixes: Error: Neither apiKey nor config.authenticator provided
- Update Stripe API version to 2026-05-27.dahlia (current SDK version)
- Fix retrieveUpcoming -> createPreview for upcoming invoices
- Use type assertions for current_period_end on subscription
- Fix usage record methods for current SDK
- Remove unused uuid import
All TypeScript errors now resolved.