feat: Authors feature backend (and upgraded deprecated mods)

docs/api/articles.md

@@ -2,7 +2,7 @@
 
 Every route that returns an article accepts the optional `format` query parameter and returns the article body in that format. Articles are stored as HTML (Tiptap output). Storage and the request bodies of `POST /v1/article/` and `PATCH /v1/article/{id}` stay HTML.
 
-`PATCH /v1/article/{id}` takes `body` and `site_id`, and nothing else. The request must contain at least one of them, and `null` for either answers `400`. Behaviour change: `{}` and `{"body": null}` used to set an empty body and now answer `400`; send `{"body": ""}` to empty a body.
+`PATCH /v1/article/{id}` takes `body`, `site_id` and `author_ids` (the whole ordered list of author ids, see `docs/api.md`), and nothing else. The request must contain at least one of them, and `null` for either answers `400`. Behaviour change: `{}` and `{"body": null}` used to set an empty body and now answer `400`; send `{"body": ""}` to empty a body.
 
 ## Routes that honour `format`
 
@@ -70,6 +70,7 @@ Breaking change: the default used to be HTML. Clients that need HTML must send `
   "body_format": "markdown",
   "is_online": true,
   "site_id": 1,
+  "authors": [],
   "created_at": "2026-03-12T09:14:00Z",
   "updated_at": "2026-09-10T07:00:00Z"
 }