Create and send a Stripe invoice to an enterprise customer.

Flow:

1. Admin enters customer email + amount + plan
2. Backend creates/retrieves Stripe customer
3. Creates an invoice with one-off line item
4. Finalizes and sends the invoice
5. Customer pays via Stripe-hosted invoice page
6. When paid, webhook (invoice.paid) activates their enterprise plan

The invoice metadata stores plan_id and auto_activate flag so the webhook handler knows which plan to assign when payment succeeds.