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/api/assets/route.ts

@@ -22,8 +22,17 @@ export async function POST(request: Request): Promise<NextResponse> {
     return NextResponse.json({ error: "Request body must be a JSON object" }, { status: 400 });
   }
 
-  const { serialNumber, assetTag, modelId, purchaseDate, warrantyEndDate, purchaseOrderId } =
-    body as Record<string, unknown>;
+  const {
+    serialNumber,
+    assetTag,
+    modelId,
+    purchaseDate,
+    warrantyEndDate,
+    purchaseOrderId,
+    cost,
+    costCurrency,
+    externalPoNumber,
+  } = body as Record<string, unknown>;
   const validation = validateCreateAssetItemInput({
     serialNumber,
     assetTag,
@@ -31,6 +40,9 @@ export async function POST(request: Request): Promise<NextResponse> {
     purchaseDate,
     warrantyEndDate,
     purchaseOrderId,
+    cost,
+    costCurrency,
+    externalPoNumber,
   });
 
   if (!validation.ok) {