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/asset-models.ts

@@ -9,6 +9,7 @@ export interface AssetModelRecord {
   manufacturer: string;
   name: string;
   specs: Record<string, unknown>;
+  warranty_months: number | null;
   created_at: string;
 }
 
@@ -17,6 +18,7 @@ export interface CreateAssetModelInput {
   manufacturer: string;
   name: string;
   specs: Record<string, unknown>;
+  warrantyMonths?: number | null;
 }
 
 export type ValidationResult =
@@ -57,6 +59,14 @@ const createAssetModelSchema = z.object({
     })
     .optional()
     .default({}),
+  // Optional standard warranty length -- see schema.prisma's
+  // AssetModel.warrantyMonths comment for what this drives.
+  warrantyMonths: z
+    .number({ invalid_type_error: "warrantyMonths must be a positive integer" })
+    .int("warrantyMonths must be a positive integer")
+    .positive("warrantyMonths must be a positive integer")
+    .optional()
+    .nullable(),
 });
 
 /**
@@ -71,6 +81,7 @@ export function validateCreateAssetModelInput(input: {
   manufacturer?: unknown;
   name?: unknown;
   specs?: unknown;
+  warrantyMonths?: unknown;
 }): ValidationResult {
   const result = createAssetModelSchema.safeParse(input);
   if (!result.success) {
@@ -86,6 +97,7 @@ function toRecord(row: {
   manufacturer: string;
   name: string;
   specs: Prisma.JsonValue;
+  warrantyMonths: number | null;
   createdAt: Date;
 }): AssetModelRecord {
   return {
@@ -94,6 +106,7 @@ function toRecord(row: {
     manufacturer: row.manufacturer,
     name: row.name,
     specs: (row.specs as Record<string, unknown>) ?? {},
+    warranty_months: row.warrantyMonths,
     created_at: row.createdAt.toISOString(),
   };
 }
@@ -201,6 +214,7 @@ export async function createAssetModel(
       // is safe here since the Zod schema already guarantees JSON-shaped
       // values.
       specs: input.specs as Prisma.InputJsonValue,
+      warrantyMonths: input.warrantyMonths ?? null,
     },
   });
   return { ok: true, value: toRecord(created) };