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/migrations/20260824081044_add_asset_item_asset_tag/migration.sql

@@ -0,0 +1,3 @@
+-- Hand-written (same deviation as other 2026-08-2x migrations -- see CLAUDE.md).
+ALTER TABLE "asset_items" ADD COLUMN "asset_tag" TEXT;
+CREATE UNIQUE INDEX "asset_items_asset_tag_key" ON "asset_items"("asset_tag");

prisma/migrations/20260824082411_asset_item_purchase_date_nullable/migration.sql

@@ -0,0 +1,2 @@
+-- Hand-written (same deviation as other 2026-08-2x migrations -- see CLAUDE.md).
+ALTER TABLE "asset_items" ALTER COLUMN "purchase_date" DROP NOT NULL;