feat: add category hero/cover photo selection (#163)

Wire up the hero_photo_id column on photo_categories that was added in
the migration but never connected. Backend routes now accept and persist
hero_photo_id on category create/update, a dedicated PUT /:id/hero
endpoint is added, and the gallery API returns hero_photo_id for each
category. Frontend EventCategoryManager shows a clickable thumbnail per
category that opens a photo picker modal. Includes EN/DE i18n keys.

frontend/src/types/index.ts

@@ -102,6 +102,7 @@ export interface PhotoCategory {
   name: string;
   slug: string;
   is_global: boolean;
+  hero_photo_id?: number | null;
 }
 
 export interface GalleryData {