Introduce Delivery date in fulfillments

🔍 Problem Statement

Shipment and return reports currently use creation dates instead of the actual delivery date, which makes reporting unclear and can misalign revenue recognition.

💡 User Story

As a finance user, I want to capture and report on the actual delivery date for shipments and returns, so that reports accurately reflect when deliveries occurred.

🎯 Definition of Done (DoD)

A feature is done when:

✔ An optional “Delivery Date” input field is added to the fulfillment creation flow on the platform.

✔ The admin sets the delivery date, which then determines the rate for all included shipments, regardless of when the fulfillment was created.

✔ The Delivery Date, when provided, is used as the primary date in:

Shipments reports
Returns reports

✔ The invoice generated should consider the delivery date rate.

✔ The unit_rate in the shipments & returns report should rely on delivery date.

✔ For returns, the Delivery Date matches the delivery date of the associated fulfillment, regardless of when the return is created.

✔ Show delivery_date in the fulfillment export at end of file.

✔ Show delivery_date in the shipments report at end of file.

✔ If the Delivery Date is not provided, the system falls back to existing date logic created-at date.