Add the ability to generate all plans defined on a Pelcro account.

🔍 Problem Statement

Developers cannot retrieve a full list of all pricing plans defined on a Pelcro account via a single API call. Plans must be fetched product-by-product, which is inefficient for integrations that need a complete plan catalogue.

💡 User Story

As a developer, I want an API endpoint that returns all plans across all products for the account, so that I can retrieve the full plan catalogue in a single call.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a Pelcro account with multiple products and plans, when a developer calls the list-all-plans endpoint, then all plans across all products are returned in a single paginated response.