feat(import): upsert ProductVendorPrice from imported PO line items

For each line item with a price and a resolved vendor, upsert a
ProductVendorPrice record (productId + vendorId → price). This keeps
the per-vendor price list in the item catalogue up to date from imports.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

App/app/(portal)/po/import/actions.ts

@@ -88,13 +88,13 @@ export async function importPo(
     let productId: string | undefined;
     if (existing) {
       productId = existing.id;
-      // Update lastPrice if we have a better price
+      // Update lastPrice / lastVendor on the product
       if (item.unitPrice > 0) {
         await db.product.update({
           where: { id: existing.id },
           data: {
             lastPrice: item.unitPrice,
-            lastVendorId: resolvedVendorId ?? undefined,
+            ...(resolvedVendorId ? { lastVendorId: resolvedVendorId } : {}),
           },
         });
       }
@@ -113,6 +113,15 @@ export async function importPo(
       productId = newProduct.id;
     }
 
+    // Upsert per-vendor price so the item catalogue reflects actual invoice prices
+    if (productId && resolvedVendorId && item.unitPrice > 0) {
+      await db.productVendorPrice.upsert({
+        where: { productId_vendorId: { productId, vendorId: resolvedVendorId } },
+        update: { price: item.unitPrice },
+        create: { productId, vendorId: resolvedVendorId, price: item.unitPrice },
+      });
+    }
+
     resolvedLineItems.push({ ...item, productId });
   }