Configurable Invoice Due Dates for Subscriptions

🔍 Problem Statement

As a billing admin, invoice due dates are auto-set ~30 days from creation and cannot be edited, so orders shipping later (e.g. a subscription created in July for an October fulfilment) appear past due before they're actually meant to be billed.

💡 User Story

As a billing admin, I want to set or override the due date on a subscription's invoice, so that future-dated orders aren't flagged past due.

🎯 Definition of Done (DoD)

• ✔ Given a subscription invoice, when an admin edits the due date in the Platform UI (or via Core API), then the new date is persisted and reflected in dunning, past-due flags, and exports.
• ✔ Works for both gateway-collected and offline-billed subscriptions; optional per-subscription default offset overrides the system default.
• ✔ This change will impact UI / API — specifically 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; gateway retry schedules remain governed by the gateway.