fix: retrofit AssetItem.model to modelId FK

AssetItem.model was a free-text string with no referential integrity.
Now a required FK to AssetModel, mirroring the Request.employeeId and
License.vendorId retrofit precedent (no backfill needed, no real
AssetItem data existed).

src/app/api/assets/route.ts

@@ -22,11 +22,11 @@ export async function POST(request: Request): Promise<NextResponse> {
     return NextResponse.json({ error: "Request body must be a JSON object" }, { status: 400 });
   }
 
-  const { serialNumber, model, purchaseDate, warrantyEndDate, purchaseOrderId } =
+  const { serialNumber, modelId, purchaseDate, warrantyEndDate, purchaseOrderId } =
     body as Record<string, unknown>;
   const validation = validateCreateAssetItemInput({
     serialNumber,
-    model,
+    modelId,
     purchaseDate,
     warrantyEndDate,
     purchaseOrderId,