6c30e2c2ed
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.