[Merged] Customizable Invoice Due Date

[Merged into Configurable Invoice Due Dates for Subscriptions]

🔍 Problem Statement

The default due date for subscription invoices is currently fixed at 30 days and cannot be customized. We need the ability to set a different default due date for all future subscription invoices.

💡 User Story

As a user, I want to configure the default due date for subscription invoices so that I can choose a payment period that fits our billing workflow.

🎯 Definition of Done (DoD)

✔ ️ Add an option to configure the default due date for subscription invoices, with options such as 30, 60, or 90 days.

✔ The selected setting should apply to all future subscription invoices.