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/api/routes/article.js

@@ -39,6 +39,7 @@ function articleErrorStatus(error) {
     if (error.message === 'Invalid unitamount') return 400;
     if (error.message === 'Unit not found') return 404;
     if (error.message === 'Invalid eans') return 400;
+    if (error.message === 'Company not found') return 404;
     if (error.message === 'Brand not found') return 404;
     if (error.message === 'Invalid weight') return 400;
     if (error.message === 'Invalid categoryIds') return 400;
@@ -1074,4 +1075,85 @@ export function register(app, basePath = '') {
             res.status(status).json({ error: error.message });
         }
     });
+
+    /**
+     * @openapi
+     * /articleIndex:
+     *   get:
+     *     tags: [Article]
+     *     summary: List article search index
+     *     description: |
+     *       Returns every article reachable via the company's category tree.
+     *       Designed for client-side search / autocomplete preload.
+     *       Auth via company access.
+     *     security:
+     *       - cookieAuth: []
+     *     responses:
+     *       200:
+     *         description: Search index
+     *         content:
+     *           application/json:
+     *             schema:
+     *               type: object
+     *               properties:
+     *                 articles:
+     *                   type: array
+     *                   items:
+     *                     type: object
+     *                     properties:
+     *                       id:
+     *                         type: integer
+     *                       sku:
+     *                         type: string
+     *                       name:
+     *                         type: string
+     *                         nullable: true
+     *                       eans:
+     *                         type: array
+     *                         items:
+     *                           type: string
+     *                       categoryIds:
+     *                         type: array
+     *                         items:
+     *                           type: integer
+     *       400:
+     *         description: No company selected
+     *         content:
+     *           application/json:
+     *             schema:
+     *               $ref: '#/components/schemas/Error'
+     *       401:
+     *         description: Unauthorized
+     *         content:
+     *           application/json:
+     *             schema:
+     *               $ref: '#/components/schemas/Error'
+     *       403:
+     *         description: Not authorized
+     *         content:
+     *           application/json:
+     *             schema:
+     *               $ref: '#/components/schemas/Error'
+     *       404:
+     *         description: Company not found
+     *         content:
+     *           application/json:
+     *             schema:
+     *               $ref: '#/components/schemas/Error'
+     */
+    app.get(`${basePath}/articleIndex`, async (req, res) => {
+        const companyId = req.user.selectedCompanyId;
+        if (companyId == null) {
+            return res.status(400).json({ error: 'No company selected' });
+        }
+
+        try {
+            const articles = await db.listSearchIndex(req.user.id, companyId);
+            res.status(200).json({ articles });
+        } catch (error) {
+            const status = articleErrorStatus(error);
+            if (status === 500) console.error('List article search index failed:', error);
+            res.status(status).json({ error: error.message });
+        }
+    });
 }