feat: Sites feature backend

docs/api/articles.md

@@ -2,6 +2,8 @@
 
 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.
+
 ## Routes that honour `format`
 
 - `GET /v1/article/`
@@ -33,6 +35,7 @@ Visibility is unchanged: anonymous and unverified callers only see online articl
 | `size` | `5`, `25`, `50` or `100` | `25` |
 | `q` | up to 200 characters, matched anywhere in the title, ignoring case and accents | none |
 | `status` | `online` or `draft` | none |
+| `site` | the `key` of a site; an unknown key gives an empty page, an empty value is `400` | none |
 | `sort` | `title`, `status`, `created_at` or `updated_at`, then `:asc` or `:desc` (lowercase, for example `title:asc`) | `updated_at:desc` |
 
 - `items` is `[]` when empty, never `null`. Each item is the article object below.
@@ -66,6 +69,7 @@ Breaking change: the default used to be HTML. Clients that need HTML must send `
   "body": "## Plan\n\nReplace the **old** deck.",
   "body_format": "markdown",
   "is_online": true,
+  "site_id": 1,
   "created_at": "2026-03-12T09:14:00Z",
   "updated_at": "2026-09-10T07:00:00Z"
 }