feat: add AssetItem Prisma model + migration

prisma/migrations/20260731090347_add_asset_item/migration.sql

@@ -0,0 +1,18 @@
+-- CreateEnum
+CREATE TYPE "AssetItemStatus" AS ENUM ('in_stock', 'assigned', 'repair', 'retired');
+
+-- CreateTable
+CREATE TABLE "asset_items" (
+    "id" UUID NOT NULL,
+    "serial_number" TEXT NOT NULL,
+    "model" TEXT NOT NULL,
+    "purchase_date" DATE NOT NULL,
+    "warranty_end_date" DATE,
+    "status" "AssetItemStatus" NOT NULL DEFAULT 'in_stock',
+    "created_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "asset_items_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "asset_items_serial_number_key" ON "asset_items"("serial_number");