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- Add stripe dependency to @groombook/api - Implement payment.ts service with: - getOrCreateStripeCustomer: look up or create Stripe customer - createPaymentIntent: create payment intent for invoice(s) - processRefund: full or partial refund - listPaymentMethods: list saved cards - attachPaymentMethod / detachPaymentMethod: manage saved cards - Lazy Stripe client initialization; graceful null return when keys not set Co-Authored-By: Paperclip <noreply@paperclip.ing>