Add AssetItem.cost/costCurrency/externalPoNumber, make import upsert-capable

Three new fields: cost (Decimal, not Float -- exact money math),
costCurrency (free text), externalPoNumber (free text PO number from an
external system, distinct from purchaseOrderId which is a real FK into
this apps own PurchaseOrder table -- external exports PO numbers are
messy free text with nothing to link to here).

importAssetItems() now upserts: a row matching an existing assetTag or
serialNumber updates that row instead of erroring on the uniqueness
conflict. Needed to backfill cost/PO onto the 613 already-imported rows
without recreating them, and sets this up for future re-syncs when the
company re-exports their ITAM report. Update only touches fields the row
actually carries a value for (blank cell = no change, not clear) --
warrantyEndDate is the one field NOT auto-recomputed from warrantyMonths
on update (unlike create), so a manually-corrected date survives a
re-sync. Placeholder junk (Not available/Pending/N/A/etc, case
-insensitive) in PO number/cost/currency cells is treated as absent.

Re-ran the full 613-row import after this: 0 created, 613 updated, 0
errors -- backfilled cost onto the 8 rows that had one and PO number onto
the 2 that had a real (non-placeholder) value, verified against the
source file.

Hand-written migration, no Vitest coverage -- same deviation as the rest
of this branch, see CLAUDE.md. tsc/lint clean.

src/app/assets/page.tsx

@@ -31,7 +31,10 @@ interface AssetItemRecord {
   purchase_date: string | null;
   warranty_end_date: string | null;
   status: "in_stock" | "assigned" | "repair" | "retired";
+  cost: number | null;
+  cost_currency: string | null;
   purchase_order_id: string | null;
+  external_po_number: string | null;
   created_at: string;
 }
 
@@ -76,6 +79,9 @@ export default function AssetsPage() {
   const [purchaseDate, setPurchaseDate] = useState("");
   const [warrantyEndDate, setWarrantyEndDate] = useState("");
   const [purchaseOrderId, setPurchaseOrderId] = useState(NO_PURCHASE_ORDER_VALUE);
+  const [cost, setCost] = useState("");
+  const [costCurrency, setCostCurrency] = useState("");
+  const [externalPoNumber, setExternalPoNumber] = useState("");
   const [submitting, setSubmitting] = useState(false);
   const [submitError, setSubmitError] = useState<string | null>(null);
 
@@ -115,7 +121,7 @@ export default function AssetsPage() {
               .join("; ") +
             (errorCount > 5 ? ` (+${errorCount - 5} lỗi khác)` : "")
           : "";
-      setImportMessage(`Đã import ${body.created} asset.` + errorSummary);
+      setImportMessage(`Tạo mới ${body.created}, cập nhật ${body.updated} asset.` + errorSummary);
       refetch();
     } catch {
       setImportStatus("error");
@@ -131,6 +137,15 @@ export default function AssetsPage() {
     const hasAssetTag = assetTag.trim() !== "";
     const hasWarrantyEndDate = warrantyEndDate.trim() !== "";
     const hasPurchaseOrderId = purchaseOrderId !== NO_PURCHASE_ORDER_VALUE;
+    const hasCost = cost.trim() !== "";
+    const hasCostCurrency = costCurrency.trim() !== "";
+    const hasExternalPoNumber = externalPoNumber.trim() !== "";
+
+    if (hasCost && Number.isNaN(Number(cost))) {
+      setSubmitError("Cost must be a number");
+      setSubmitting(false);
+      return;
+    }
 
     try {
       const response = await fetch("/api/assets", {
@@ -146,6 +161,9 @@ export default function AssetsPage() {
           ...(hasAssetTag ? { assetTag } : {}),
           ...(hasWarrantyEndDate ? { warrantyEndDate } : {}),
           ...(hasPurchaseOrderId ? { purchaseOrderId } : {}),
+          ...(hasCost ? { cost: Number(cost) } : {}),
+          ...(hasCostCurrency ? { costCurrency } : {}),
+          ...(hasExternalPoNumber ? { externalPoNumber } : {}),
         }),
       });
 
@@ -174,6 +192,9 @@ export default function AssetsPage() {
       setPurchaseDate("");
       setWarrantyEndDate("");
       setPurchaseOrderId(NO_PURCHASE_ORDER_VALUE);
+      setCost("");
+      setCostCurrency("");
+      setExternalPoNumber("");
       refetch();
     } catch {
       setSubmitError("Request failed");
@@ -202,7 +223,9 @@ export default function AssetsPage() {
                 <TableHead>Purchase Date</TableHead>
                 <TableHead>Warranty End Date</TableHead>
                 <TableHead>Status</TableHead>
+                <TableHead>Cost</TableHead>
                 <TableHead>Purchase Order ID</TableHead>
+                <TableHead>External PO Number</TableHead>
                 <TableHead>Created At</TableHead>
               </TableRow>
             </TableHeader>
@@ -216,7 +239,13 @@ export default function AssetsPage() {
                   <TableCell>{assetItem.purchase_date ?? "-"}</TableCell>
                   <TableCell>{assetItem.warranty_end_date ?? "-"}</TableCell>
                   <TableCell>{assetItem.status}</TableCell>
+                  <TableCell>
+                    {assetItem.cost !== null
+                      ? `${assetItem.cost}${assetItem.cost_currency ? " " + assetItem.cost_currency : ""}`
+                      : "-"}
+                  </TableCell>
                   <TableCell>{assetItem.purchase_order_id ?? "-"}</TableCell>
+                  <TableCell>{assetItem.external_po_number ?? "-"}</TableCell>
                   <TableCell>{assetItem.created_at}</TableCell>
                 </TableRow>
               ))}
@@ -233,11 +262,14 @@ export default function AssetsPage() {
           <a href="/asset-models" className="underline">
             /asset-models
           </a>
-          ), <code>purchase date</code> (bắt buộc -- cũng nhận <code>order received</code>).
+          ), <code>purchase date</code> (không bắt buộc -- cũng nhận <code>order received</code>).
           Tuỳ chọn: <code>asset tag</code>, <code>warranty end date</code> (bỏ trống thì tự tính
           từ warranty của model), <code>status</code>/<code>state</code> (nhận{" "}
           <code>In use</code> → assigned, <code>In stock</code> → in_stock, bỏ trống thì mặc định
-          in_stock), <code>purchase order id</code>.
+          in_stock), <code>cost</code>, <code>cost currency</code>,{" "}
+          <code>po number</code> (text tự do, khác <code>purchase order id</code> là UUID trong
+          hệ thống). Dòng có <code>asset tag</code> hoặc <code>serial number</code> đã tồn tại sẽ{" "}
+          <b>cập nhật</b> thay vì báo lỗi trùng -- import lại file mới để đồng bộ dữ liệu.
         </p>
         <div className="flex flex-wrap items-center gap-3">
           <a href="/api/assets/export">
@@ -348,6 +380,38 @@ export default function AssetsPage() {
             </Select>
           </div>
 
+          <div className="flex flex-col gap-2">
+            <Label htmlFor="cost">Cost (optional)</Label>
+            <Input
+              id="cost"
+              type="number"
+              min={0}
+              step="0.01"
+              value={cost}
+              onChange={(event) => setCost(event.target.value)}
+            />
+          </div>
+
+          <div className="flex flex-col gap-2">
+            <Label htmlFor="costCurrency">Cost Currency (optional)</Label>
+            <Input
+              id="costCurrency"
+              value={costCurrency}
+              onChange={(event) => setCostCurrency(event.target.value)}
+              placeholder="e.g. VND, USD"
+            />
+          </div>
+
+          <div className="flex flex-col gap-2">
+            <Label htmlFor="externalPoNumber">External PO Number (optional)</Label>
+            <Input
+              id="externalPoNumber"
+              value={externalPoNumber}
+              onChange={(event) => setExternalPoNumber(event.target.value)}
+              placeholder="PO number from an external system, if any"
+            />
+          </div>
+
           {submitError ? <p className="text-sm text-destructive">{submitError}</p> : null}
 
           <Button type="submit" disabled={submitting}>