fix(admin/feedback): use correct event id when rendering photo thumbnails

- Replace undefined eventId with route param id to build admin thumbnail URL
- Fixes runtime ReferenceError on /admin/events/:id/feedback when opening Feedback tab

Refs: #19

frontend/src/pages/admin/EventFeedbackPage.tsx

@@ -254,7 +254,7 @@ export const EventFeedbackPage: React.FC = () => {
                     {item.photo_id && (
                       <div className="w-16 h-16 overflow-hidden rounded">
                         <AdminAuthenticatedImage
-                          src={`/api/admin/photos/${eventId}/thumbnail/${item.photo_id}`}
+                          src={`/api/admin/photos/${id}/thumbnail/${item.photo_id}`}
                           alt={item.filename || 'Photo'}
                           className="w-16 h-16 object-cover rounded"
                         />