Subscription and customer creation optimization

🔍 Problem Statement

Creating a subscription with a new customer requires multiple sequential API calls (create customer, create address, create payment method, create subscription). This multi-step process is slow, error-prone, and creates orphaned records if any step fails midway.

💡 User Story

As a developer, I want a single API call that creates a customer and subscription together, so that the entire operation is atomic — reducing latency, complexity, and the risk of partial/orphaned records.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscription creation request with inline customer data, when the API call is made, then the customer, address, payment method, and subscription are created atomically — and if any step fails, the entire operation is rolled back.