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/services/categories.service.ts

@@ -6,6 +6,7 @@ export interface PhotoCategory {
   slug: string;
   is_global: boolean;
   event_id: number | null;
+  hero_photo_id?: number | null;
   created_at: string;
 }
 
@@ -41,6 +42,12 @@ export const categoriesService = {
     return response.data;
   },
 
+  // Set category hero photo (#163)
+  async setCategoryHeroPhoto(id: number, heroPhotoId: number | null): Promise<PhotoCategory> {
+    const response = await api.put<PhotoCategory>(`/admin/categories/${id}/hero`, { hero_photo_id: heroPhotoId });
+    return response.data;
+  },
+
   // Delete a category
   async deleteCategory(id: number): Promise<void> {
     await api.delete(`/admin/categories/${id}`);