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/add-image.md

@@ -0,0 +1,44 @@
+# `POST /category/{id}/image`
+
+Protected. Requires a valid session cookie (`credentials: 'include'`).
+
+Add category image
+
+Accepts a full-size AVIF body (`Content-Type: image/avif`).
+Full: stored as-is when already AVIF and ≤1000×1000; otherwise scaled to max 1000×1000.
+Thumb always scaled to max 200×200. Stores hash/thumbhash, links `categorypic`.
+Auth like GET /category/tree — user must be authorized for the category's company.
+
+
+## Path parameters
+
+| Param | Required | Type | Description |
+|-------|----------|------|-------------|
+| `id` | yes | integer | Category id |
+
+## Body
+
+Raw binary (`image/avif`).
+
+## Response `201`
+
+Image stored and linked
+
+```json
+{
+  "category": { … }
+}
+```
+
+## Errors
+
+| Status | Description |
+|--------|-------------|
+| 400 | Missing body or invalid image — `{"error":"…"}` |
+| 401 | Unauthorized — `{"error":"…"}` |
+| 403 | Not authorized — `{"error":"…"}` |
+| 404 | Category not found — `{"error":"…"}` |
+
+---
+
+_Generated from `@openapi` JSDoc. Do not edit by hand — run `npm run docs:api`._