feat: add article search index endpoint and category export.zip

- Add GET /api/articleIndex returning the article search index (listSearchIndex)
- Add GET /api/category/export.zip returning a kategorien.zip with a
  Windows-1252 CSV (depth-first, Ebene N columns) plus one AVIF per category image
  (content-addressed, deduplicated) using jszip

src/db/article.js

@@ -59,3 +59,8 @@ export async function setCategoryIdsBySku(googleAuthId, sku, categoryIds) {
     const store = await getStore();
     return store.article.setCategoryIdsBySku(googleAuthId, sku, categoryIds);
 }
+
+export async function listSearchIndex(googleAuthId, companyId) {
+    const store = await getStore();
+    return store.article.listSearchIndex(googleAuthId, companyId);
+}