🔍 Problem Statement
As a billing admin, subscription invoice due dates are hard-coded to ~30 days from creation and cannot be changed — neither as an account-wide default nor on an individual invoice — so orders shipping later (e.g. a July subscription for an October fulfilment) appear past due before they are meant to be billed.
💡 User Story
As a billing admin, I want to configure the default due date for subscription invoices and override it on an individual invoice, so that the payment period fits our billing workflow and future-dated orders aren't flagged past due.
🎯 Definition of Done (DoD)
- ✔ Given Settings, when an admin selects a default due-date offset (e.g. 30 / 60 / 90 days), then it applies to all future subscription invoices.
- ✔ Given a subscription invoice, when an admin edits its due date in the Platform UI or via the Core API, then the override is persisted and reflected in dunning, past-due flags, and exports.
- ✔ Works for both gateway-collected and offline-billed subscriptions.
- ✔ This change will impact UI / API — specifically Settings and the Subscription/Invoice detail page (Platform UI), and the subscription-invoice resource (Core API).
- ✔ Limitations: due date cannot be set earlier than the invoice issue date; existing invoices keep their due date when the default changes; gateway retry schedules remain governed by the gateway.
