Add missing subscription fields to the API response

🔍 Problem Statement

The Pelcro subscription API response is missing fields that clients need for integration and reporting (e.g. plan name, product name, custom fields). This forces developers to make additional API calls to assemble complete subscription data.

💡 User Story

As a developer, I want all relevant subscription fields included in the API response, so that I can retrieve complete subscription data in a single call without additional lookups.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscription API request (list or get), when the response is returned, then it includes all commonly needed fields (plan name, product name, custom fields, billing details) without requiring additional API calls.