Rename the PurchaseOrder property "SpecialInstructions" to "Motivations". Scope STRICTLY to the PurchaseOrder side: the PurchaseOrder entity, its DTO, its repository, its base interface, its service, and its detail model.

CRITICAL precision rules:
- Do NOT touch PurchaseRequisition — it has its OWN SpecialInstructions property that MUST stay named SpecialInstructions.
- In the PurchaseOrder service, where a PurchaseOrder is built FROM a requisition, the assignment currently reads "SpecialInstructions = requisition.SpecialInstructions". It must become "Motivations = requisition.SpecialInstructions" — rename ONLY the left-hand PurchaseOrder property; the right-hand "requisition.SpecialInstructions" stays unchanged (that is the PurchaseRequisition source).
- Only rename SpecialInstructions where it refers to the PurchaseOrder's own property. Make no other changes.
