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).

prisma/schema.prisma

@@ -118,9 +118,25 @@ enum AssetItemStatus {
 model AssetItem {
   id                 String               @id @default(uuid()) @db.Uuid
   serialNumber       String               @unique @map("serial_number")
+  // Company-assigned physical tag (e.g. "DANLAP001", printed on a sticker
+  // on the device) -- 2026-08-24, added when bulk-importing the real
+  // ServiceNow ITAM export (see docs/asset-license-management.md). Was
+  // only on InventoryScan before (the office-move walk feature); this is
+  // the same concept but on the actual asset record, not a move-day scan.
+  // Nullable (a manually-entered new purchase may not have one yet) but
+  // unique when present -- the real export had 613/613 unique, non-null
+  // tags, same integrity guarantee as serialNumber.
+  assetTag           String?              @unique @map("asset_tag")
   modelId            String               @map("model_id") @db.Uuid
   model              AssetModel           @relation(fields: [modelId], references: [id])
-  purchaseDate       DateTime             @map("purchase_date") @db.Date
+  // Nullable as of 2026-08-24 (was NOT NULL) -- bulk-importing the real
+  // ITAM export, some legacy rows have no known purchase date at all (no
+  // "Order received" value in the source). Manual /assets create form
+  // still requires it (a NEW purchase should always have a known date);
+  // this only relaxes the DB/import path for historical data. Also means
+  // AssetModel.warrantyMonths auto-fill (see createAssetItem()) can't
+  // compute anything for these rows -- warrantyEndDate stays null too.
+  purchaseDate       DateTime?            @map("purchase_date") @db.Date
   warrantyEndDate    DateTime?            @map("warranty_end_date") @db.Date
   status             AssetItemStatus      @default(in_stock)
   purchaseOrderId    String?              @map("purchase_order_id") @db.Uuid