Wrong Frontend Calculation in Preview during Create Subscription flow

Problem

• As we continue to add more entities within a subscription, the calculation logic on the frontend is increasingly diverging from the backend.
• This results in inconsistent values being displayed to users and introduces room for errors, especially as the logic becomes more complex.

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Goal

Define a single source of truth for subscription-related calculations that can be consistently applied across both frontend and backend. This could involve introducing a shared rules engine (e.g., JSON-based rules or configuration-driven approach) to ensure all calculations remain accurate, maintainable, and easy to extend in the future. Open for discussion.