feat: add License.purchaseOrderId (nullable FK)

Nullable FK from License to PurchaseOrder for supply-side traceability
(which PO a license came in through), mirroring AssetItem.purchaseOrderId's
just-merged pattern exactly. Separate commit from the earlier
License.vendorId retrofit per the project's 1-commit-per-business-change
convention.

prisma/migrations/20260803001538_add_license_purchase_order_id/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "licenses" ADD COLUMN     "purchase_order_id" UUID;
+
+-- AddForeignKey
+ALTER TABLE "licenses" ADD CONSTRAINT "licenses_purchase_order_id_fkey" FOREIGN KEY ("purchase_order_id") REFERENCES "purchase_orders"("id") ON DELETE SET NULL ON UPDATE CASCADE;