Add optional hub field to Newsroom type

Mirrors the additive `hub` field introduced by prezly/prezly#18567,
which exposes the parent HUB and active siblings (each with `cultures`)
on child-newsroom API responses so themes can render cross-site language
pickers. Field is marked optional so the SDK release is not gated on
backend deploy order.

Author: JesseWynants

src/types/Newsroom.ts

@@ -81,6 +81,7 @@ export interface NewsroomRef {
 
 export interface Newsroom extends NewsroomRef {
     is_hub: boolean;
+    hub?: { parent: Newsroom.HubPeer; siblings: Newsroom.HubPeer[] } | null;
     // extended details
     cultures: CultureRef[];
     campaigns_number: number;
@@ -140,6 +141,10 @@ export interface Newsroom extends NewsroomRef {
 }
 
 export namespace Newsroom {
+    export interface HubPeer extends NewsroomRef {
+        cultures: CultureRef[];
+    }
+
     export enum Status {
         ACTIVE = 'active', // i.e. "Live"
         INACTIVE = 'inactive', // i.e. "Not live"