fix: resolve feedback validation issues from GitHub issue #16

- Fixed 400 Bad Request error when submitting feedback with name/email required
- Updated backend validation to properly handle empty/undefined name/email fields
- Modified frontend components to send undefined instead of empty strings when fields are not provided
- Fixed thumbnail display issue in moderation view by using correct admin API endpoints
- Updated FeedbackModerationPanel and EventFeedbackPage to display thumbnails correctly

The issue was caused by the validation logic treating empty strings differently than undefined values.
Frontend components now properly send undefined when name/email are not provided, and the backend
validation correctly handles both cases.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/admin/FeedbackModerationPanel.tsx

@@ -126,11 +126,22 @@ export const FeedbackModerationPanel: React.FC<FeedbackModerationPanelProps> = (
                             )}
                           </span>
                         </div>
-                        <p className="mt-1 text-sm text-neutral-700">{item.comment}</p>
-                        {item.photo_filename && (
-                          <p className="mt-1 text-xs text-neutral-500">
-                            {t('feedback.onPhoto', 'On photo')}: {item.photo_filename}
-                          </p>
+                        <p className="mt-1 text-sm text-neutral-700">{item.comment_text || item.comment}</p>
+                        {item.photo_id && (
+                          <div className="mt-2 flex items-center gap-2">
+                            <img 
+                              src={`/api/admin/photos/${eventId}/thumbnail/${item.photo_id}`}
+                              alt={item.filename || 'Photo'}
+                              className="w-16 h-16 object-cover rounded"
+                              onError={(e) => {
+                                // Hide image if thumbnail fails to load
+                                (e.target as HTMLImageElement).style.display = 'none';
+                              }}
+                            />
+                            <p className="text-xs text-neutral-500">
+                              {t('feedback.onPhoto', 'On photo')}: {item.filename || item.photo_filename || `#${item.photo_id}`}
+                            </p>
+                          </div>
                         )}
                       </div>
                     </div>