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/util/kategorienCsv.js

@@ -0,0 +1,76 @@
+export const ROOT_CATEGORY_NAME = 'Home';
+
+/** Escape a value for a CSV field ("..." double-quoting per RFC 4180). */
+function csvField(value) {
+    const text = value == null ? '' : String(value);
+    return /[";\n\r]/.test(text) ? `"${text.replace(/"/g, '""')}"` : text;
+}
+
+function categoryDisplayName(name, categoryId, rootCategoryId) {
+    if (rootCategoryId != null && String(categoryId) === String(rootCategoryId)) {
+        return ROOT_CATEGORY_NAME;
+    }
+    return (name ?? '').trim();
+}
+
+/**
+ * Build the kategorien.csv string from the flattened category tree returned by
+ * `getTreeForCompany`. Each row carries FullHash;Id and one name cell in the
+ * `Ebene N` column matching the category's tree depth (root = Ebene 1).
+ * Mirrors the frontend's client-side export format exactly.
+ */
+export function buildKategorienCsv(tree, rootCategoryId) {
+    if (!Array.isArray(tree) || tree.length === 0) return '';
+
+    const children = new Map();
+    const keyOf = (parentId) =>
+        parentId == null || parentId === 0 ? 'ROOT' : String(parentId);
+    for (const node of tree) {
+        const key = keyOf(node.parentId);
+        if (!children.has(key)) children.set(key, []);
+        children.get(key).push(node);
+    }
+    children.forEach((arr) =>
+        arr.sort((a, b) => (a.sort ?? 0) - (b.sort ?? 0) || a.id - b.id),
+    );
+
+    const root = tree.find((n) => keyOf(n.parentId) === 'ROOT');
+    const ordered = [];
+    const visited = new Set();
+    const walk = (node, depth) => {
+        if (node == null || visited.has(String(node.id))) return;
+        visited.add(String(node.id));
+        ordered.push({ node, depth });
+        (children.get(String(node.id)) ?? []).forEach((kid) => walk(kid, depth + 1));
+    };
+    walk(root, 0);
+
+    const maxDepth = ordered.reduce((m, item) => Math.max(m, item.depth), 0);
+    const levelHeaders = Array.from({ length: maxDepth + 1 }, (_, i) => `Ebene ${i + 1}`);
+    const headers = ['FullHash', 'Id', ...levelHeaders];
+
+    const lines = [
+        'sep=;',
+        headers.map(csvField).join(';'),
+        ...ordered.map(({ node, depth }) => {
+            const name = categoryDisplayName(node.name, node.id, rootCategoryId);
+            return [
+                csvField(node.fullHash ?? ''),
+                csvField(node.id),
+                ...levelHeaders.map((_, index) => (index === depth ? csvField(name) : '')),
+            ].join(';');
+        }),
+    ];
+
+    return lines.join('\r\n');
+}
+
+/** Encode a string as Windows-1252 bytes (single-byte; out-of-range → '?'). */
+export function toWindows1252(str) {
+    const bytes = Buffer.alloc(str.length);
+    for (let i = 0; i < str.length; i++) {
+        const code = str.charCodeAt(i);
+        bytes[i] = code <= 0xff ? code : 0x3f; // '?'
+    }
+    return bytes;
+}