Add AssetItem.assetTag, status-aware bulk import, nullable purchaseDate

Prep work for importing a real company ITAM export (613 real assets):

- AssetItem.assetTag (nullable, unique) -- company physical tag, was only
  on InventoryScan (move feature) before, now on the actual asset record.
- AssetItem.purchaseDate is now nullable (was required) -- some legacy
  rows genuinely have no known purchase date. Manual /assets create form
  still requires it in the HTML input; this only relaxes the DB/import
  path for historical data.
- importAssetItems() accepts a status column now (free-text "In use"/
  "In stock"/etc mapped via IMPORT_STATUS_MAP), unlike manual creation
  which always defaults in_stock -- bulk-imported inventory is usually
  already deployed, not freshly purchased, so forcing in_stock would
  misrepresent reality.

Hand-written migrations (asset_tag, purchase_date nullable), no Vitest
coverage -- same deviation as the rest of this branch, see CLAUDE.md.
tsc/lint clean.

Actually imported 613/613 real AssetItem rows from the export with 0
errors after this (not committed here -- thats data, not code; see
CLAUDE.md for the seeding/import process and whats deliberately out of
scope this round: Cost, PO number, employee Assignment).

src/app/assets/page.tsx

@@ -26,8 +26,9 @@ import {
 interface AssetItemRecord {
   id: string;
   serial_number: string;
+  asset_tag: string | null;
   model_id: string;
-  purchase_date: string;
+  purchase_date: string | null;
   warranty_end_date: string | null;
   status: "in_stock" | "assigned" | "repair" | "retired";
   purchase_order_id: string | null;
@@ -70,6 +71,7 @@ export default function AssetsPage() {
   const { data: purchaseOrders } = useApiList<PurchaseOrderRecord>("/api/purchase-orders");
 
   const [serialNumber, setSerialNumber] = useState("");
+  const [assetTag, setAssetTag] = useState("");
   const [modelId, setModelId] = useState("");
   const [purchaseDate, setPurchaseDate] = useState("");
   const [warrantyEndDate, setWarrantyEndDate] = useState("");
@@ -126,6 +128,7 @@ export default function AssetsPage() {
     setSubmitting(true);
     setSubmitError(null);
 
+    const hasAssetTag = assetTag.trim() !== "";
     const hasWarrantyEndDate = warrantyEndDate.trim() !== "";
     const hasPurchaseOrderId = purchaseOrderId !== NO_PURCHASE_ORDER_VALUE;
 
@@ -140,6 +143,7 @@ export default function AssetsPage() {
           // Omit optional keys entirely when left blank, rather than
           // sending null — matches the create-form pattern used on other
           // pages (e.g. purchase-orders page's receivedAt/receivedById).
+          ...(hasAssetTag ? { assetTag } : {}),
           ...(hasWarrantyEndDate ? { warrantyEndDate } : {}),
           ...(hasPurchaseOrderId ? { purchaseOrderId } : {}),
         }),
@@ -165,6 +169,7 @@ export default function AssetsPage() {
       }
 
       setSerialNumber("");
+      setAssetTag("");
       setModelId("");
       setPurchaseDate("");
       setWarrantyEndDate("");
@@ -192,6 +197,7 @@ export default function AssetsPage() {
               <TableRow>
                 <TableHead>ID</TableHead>
                 <TableHead>Serial Number</TableHead>
+                <TableHead>Asset Tag</TableHead>
                 <TableHead>Model ID</TableHead>
                 <TableHead>Purchase Date</TableHead>
                 <TableHead>Warranty End Date</TableHead>
@@ -205,8 +211,9 @@ export default function AssetsPage() {
                 <TableRow key={assetItem.id}>
                   <TableCell>{assetItem.id}</TableCell>
                   <TableCell>{assetItem.serial_number}</TableCell>
+                  <TableCell>{assetItem.asset_tag ?? "-"}</TableCell>
                   <TableCell>{assetItem.model_id}</TableCell>
-                  <TableCell>{assetItem.purchase_date}</TableCell>
+                  <TableCell>{assetItem.purchase_date ?? "-"}</TableCell>
                   <TableCell>{assetItem.warranty_end_date ?? "-"}</TableCell>
                   <TableCell>{assetItem.status}</TableCell>
                   <TableCell>{assetItem.purchase_order_id ?? "-"}</TableCell>
@@ -226,8 +233,11 @@ export default function AssetsPage() {
           <a href="/asset-models" className="underline">
             /asset-models
           </a>
-          ), <code>purchase date</code> (bắt buộc), <code>warranty end date</code>,{" "}
-          <code>purchase order id</code> (tuỳ chọn -- bỏ trống thì tự tính từ warranty của model).
+          ), <code>purchase date</code> (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>.
         </p>
         <div className="flex flex-wrap items-center gap-3">
           <a href="/api/assets/export">
@@ -275,6 +285,15 @@ export default function AssetsPage() {
             />
           </div>
 
+          <div className="flex flex-col gap-2">
+            <Label htmlFor="assetTag">Asset Tag (optional)</Label>
+            <Input
+              id="assetTag"
+              value={assetTag}
+              onChange={(event) => setAssetTag(event.target.value)}
+            />
+          </div>
+
           <div className="flex flex-col gap-2">
             <Label htmlFor="modelId">Model</Label>
             <Select value={modelId} onValueChange={setModelId} required>