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

docs/api/routes/category/export.zip.md

@@ -0,0 +1,29 @@
+# `GET /category/export.zip`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Export categories as ZIP
+
+Builds a `kategorien.zip` for the authenticated user's selected company.
+The ZIP contains `kategorien.csv` (semicolon-separated, Windows-1252,
+depth-first in sort order with an `Ebene N` name column per tree depth)
+plus one AVIF entry per category image, named `<fullHash>.avif`
+(content-addressed, so shared hashes are deduplicated).
+
+
+## Response `200`
+
+ZIP archive
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | No company selected — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+| 403 | Not authorized — `{"error":"…"}` |
+| 404 | Company not found — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._