Require submitted purchase orders for invoice registration

[dotnetspark]

Summary

Invoice registration currently allows linking an invoice to a purchase order even when the purchase order is still in DRAFT state.

Expected behavior

Invoice registration should require the linked purchase order to be at least SUBMITTED.

Scope

• reject invoice creation against draft purchase orders
• add contract/integration/unit coverage
• update owned specs/docs/changelog to reflect the rule

Notes

This closes the previously identified business-rule gap where purchase-order existence and vendor coherence were enforced, but purchase-order readiness for invoicing was not.