Add PUT /category/order to reorder sibling categories.

Wires up existing category.order store method (validates categoryIds are
exactly the current children of parentId, sets each child sort to its
index in one transaction) behind an authenticated HTTP route, plus
@openapi docs and regenerated API docs.

README.md

@@ -53,6 +53,7 @@ That writes `docs/api/openapi.json` and per-route Markdown under `docs/api/route
 | Methode | Route | Store-Methode |
 | --- | --- | --- |
 | `PATCH` | `/api/category/:id` | `category.rename` (Name + Slug in Firmensprache) |
+| `PUT` | `/api/category/order` | `category.order` (Geschwister-Sortierung unter einem Elternteil) |
 
 ## Users