Create Invoice

Create a Stripe invoice for token purchase

This endpoint implements the complete invoice flow:

1. Creates Stripe Invoice (open) with line item amount
2. Metadata: client_id, user_id, token_amount
3. Create local Payment record (status=PENDING, link to Invoice id)
4. Optionally emails hosted invoice URL to customer's billing email

The token granting happens automatically via webhook when the invoice is paid.