feat: add AssetItem Prisma model + migration

prisma/schema.prisma

@@ -63,6 +63,35 @@ model Employee {
   @@map("employees")
 }
 
+// Fixed, closed set of 4 values tied to the domain (see docs/
+// asset-license-management.md's "Data model (sơ bộ)" table) — native
+// Postgres enum, same rationale as RequestType above.
+enum AssetItemStatus {
+  in_stock
+  assigned
+  repair
+  retired
+}
+
+// Hardware inventory — see docs/asset-license-management.md's "Data model
+// (sơ bộ)" table (serial number, model, ngày mua, bảo hành, trạng thái).
+// No Vendor/Assignment relation yet — those entities don't exist yet
+// (see "Việc còn dở" in CLAUDE.md), so this is a standalone entity for now,
+// same starting shape as Employee before Request grew an FK to it.
+// warrantyEndDate is nullable: not every asset has known warranty terms at
+// intake time (smallest sensible assumption — doesn't block creation).
+model AssetItem {
+  id              String          @id @default(uuid()) @db.Uuid
+  serialNumber    String          @unique @map("serial_number")
+  model           String
+  purchaseDate    DateTime        @map("purchase_date") @db.Date
+  warrantyEndDate DateTime?       @map("warranty_end_date") @db.Date
+  status          AssetItemStatus @default(in_stock)
+  createdAt       DateTime        @default(now()) @map("created_at") @db.Timestamptz(6)
+
+  @@map("asset_items")
+}
+
 // Minimal internal auth accounts — NOT company SSO (deliberately ruled out,
 // see CLAUDE.md / auth decision notes). A handful of trusted IT staff
 // accounts, created by hand via scripts/create-user.ts, no self-registration.