Convert single-step registration into a two-step flow:
- Step 1: Registration info → saves to JSON + Google Sheet with awaiting_payment status
- Step 2: Accommodation selection + price summary → creates Mollie checkout → redirects
On payment completion, Mollie webhook updates status, assigns bed on booking sheet,
sends confirmation email, and adds to Listmonk.
New files: api/booking-sheet.js, payment-success.html
New routes: /api/create-checkout-session, /api/mollie/webhook, /api/payment-status
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
