Auto-fill AssetItem.warrantyEndDate from AssetModel.warrantyMonths

AssetModel gains an optional warrantyMonths (standard vendor warranty
length). createAssetItem() now defaults warrantyEndDate to purchaseDate +
warrantyMonths (month-end clamped, e.g. Dec 31 + 2mo -> Feb 28, not Mar 3)
whenever the caller omits warrantyEndDate -- still overridable per item
for batches with extended/different warranty terms. /asset-models UI gets
an input for the new field; /assets needs no change since it already
omits warrantyEndDate from the payload when left blank.

Hand-written migration + manual smoke test (tsx script, not Vitest),
same deviation as the other 2026-08-2x work in this branch -- see
CLAUDE.md. tsc/lint clean.

src/lib/assets.ts

@@ -136,16 +136,53 @@ export type CreateAssetItemResult =
  * createPurchaseOrder() pattern for disambiguating between two FKs on the
  * same model.
  */
+// Adds `months` calendar months to `date`, clamping to the last day of the
+// target month rather than letting JS's native Date rollover push into the
+// month after (e.g. Jan 31 + 1 month must land on Feb 28/29, not Mar
+// 2/3 -- `date.setMonth()` alone does the latter). UTC throughout: Prisma
+// @db.Date fields round-trip as UTC-midnight Dates (see dateString()
+// above), and doing this in UTC avoids the host server's local timezone
+// shifting the day near midnight.
+function addMonthsClamped(date: Date, months: number): Date {
+  const targetMonthIndex = date.getUTCMonth() + months;
+  const daysInTargetMonth = new Date(
+    Date.UTC(date.getUTCFullYear(), targetMonthIndex + 1, 0)
+  ).getUTCDate();
+  return new Date(
+    Date.UTC(
+      date.getUTCFullYear(),
+      targetMonthIndex,
+      Math.min(date.getUTCDate(), daysInTargetMonth)
+    )
+  );
+}
+
 export async function createAssetItem(
   input: CreateAssetItemInput
 ): Promise<CreateAssetItemResult> {
   try {
+    // warrantyEndDate defaults from the model's standard warranty term
+    // (AssetModel.warrantyMonths) when the caller doesn't supply one
+    // explicitly -- see schema.prisma's AssetModel.warrantyMonths comment
+    // for why this stays a suggestion (still overridable per item) rather
+    // than a hard-computed column. A modelId that doesn't exist is left for
+    // the insert below's own P2003 handling, same as before this lookup was
+    // added -- this just also opportunistically uses the model row if it's
+    // there.
+    let warrantyEndDate = input.warrantyEndDate ?? null;
+    if (!warrantyEndDate) {
+      const model = await prisma.assetModel.findUnique({ where: { id: input.modelId } });
+      if (model?.warrantyMonths) {
+        warrantyEndDate = addMonthsClamped(input.purchaseDate, model.warrantyMonths);
+      }
+    }
+
     const created = await prisma.assetItem.create({
       data: {
         serialNumber: input.serialNumber,
         modelId: input.modelId,
         purchaseDate: input.purchaseDate,
-        warrantyEndDate: input.warrantyEndDate ?? null,
+        warrantyEndDate,
         purchaseOrderId: input.purchaseOrderId ?? null,
       },
     });