Add POST /category/:id/move to relocate a category.

Wires up the existing category.move store method (validates source and
target in the same company, rejects self/descendant cycles, assigns the
next sibling sort) behind an authenticated HTTP route, plus @openapi
docs and regenerated API docs.

README.md

@@ -54,6 +54,7 @@ That writes `docs/api/openapi.json` and per-route Markdown under `docs/api/route
 | --- | --- | --- |
 | `PATCH` | `/api/category/:id` | `category.rename` (Name + Slug in Firmensprache) |
 | `PUT` | `/api/category/order` | `category.order` (Geschwister-Sortierung unter einem Elternteil) |
+| `POST` | `/api/category/:id/move` | `category.move` (Kategorie unter neues Elternteil verschieben) |
 
 ## Users