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/api/assets/route.ts

@@ -22,10 +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, modelId, purchaseDate, warrantyEndDate, purchaseOrderId } =
+  const { serialNumber, assetTag, modelId, purchaseDate, warrantyEndDate, purchaseOrderId } =
     body as Record<string, unknown>;
   const validation = validateCreateAssetItemInput({
     serialNumber,
+    assetTag,
     modelId,
     purchaseDate,
     warrantyEndDate,