Add Proration Calculation Preview API for Subscription Updates

🔍 Problem Statement
As a business, I have no way of calculating proration in advance of making a subscription update through the Pelcro API, which results in uncertainty about charges and prevents me from displaying accurate costs to my customers before they commit to the change.

💡 User Story
As a Pelcro API user, I want an endpoint that calculates proration for a subscription change in advance, so that I can show customers a clear summary of the charges or credits they will incur before confirming the update.

🎯 Definition of Done (DoD)
✔ Given I want to preview the impact of a subscription update, when I call the new proration calculation API with the subscription ID and intended changes, then I should receive a response with a detailed breakdown of:

• Upcoming invoice amount (including proration).

• Credits applied, if any.

• Net change due.
  ✔ The endpoint should not persist changes—it should only return a calculation.
  ✔ Documentation is updated with request/response schema and usage examples.
  ✔ SDKs and Postman collection updated to include the new endpoint.