feat: add public inventory catalog boundary

packages/contracts/public-inventory.js

@@ -0,0 +1,19 @@
+function toIsoString(value) {
+  return value instanceof Date ? value.toISOString() : value;
+}
+
+export function toPublicInventoryItemDto(item) {
+  return {
+    id: item.id,
+    productId: item.productId,
+    productName: item.productName,
+    productSlug: item.productSlug,
+    categoryId: item.categoryId,
+    conditionId: item.conditionId,
+    askingPriceMinor: item.askingPriceMinor,
+    listingStatus: item.listingStatus,
+    listingNotes: item.listingNotes ?? null,
+    createdAt: toIsoString(item.createdAt),
+    updatedAt: toIsoString(item.updatedAt),
+  };
+}