fix: resolve multiple feedback management issues

- Hide "Manage Feedback" button when feedback is disabled for an event
- Fix 500 error on feedback API endpoint by adding null-safe operators
- Fix TypeError on analytics page by calculating average_rating in backend
- Fix password validation for event creation by properly awaiting async validation
- Add proper null checks and fallbacks for feedback statistics

These fixes ensure:
- Date passwords like "19.07.2025" work with simple password complexity settings
- Feedback management page loads without errors
- Analytics display correctly even with no feedback data

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

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

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -349,14 +349,16 @@ export const EventDetailsPage: React.FC = () => {
                     {t('common.edit')}
                   </Button>
                 )}
-                <Button
-                  variant="outline"
-                  size="sm"
-                  leftIcon={<MessageSquare className="w-4 h-4" />}
-                  onClick={() => navigate(`/admin/events/${id}/feedback`)}
-                >
-                  {t('feedback.manage', 'Manage Feedback')}
-                </Button>
+                {feedbackSettings?.feedback_enabled && (
+                  <Button
+                    variant="outline"
+                    size="sm"
+                    leftIcon={<MessageSquare className="w-4 h-4" />}
+                    onClick={() => navigate(`/admin/events/${id}/feedback`)}
+                  >
+                    {t('feedback.manage', 'Manage Feedback')}
+                  </Button>
+                )}
               </>
             )}
             {event.share_link && (

frontend/src/pages/admin/EventFeedbackPage.tsx

@@ -392,7 +392,7 @@ export const EventFeedbackPage: React.FC = () => {
                     <div className="flex items-center gap-3 mb-2">
                       <Star className="w-8 h-8 text-yellow-500" />
                       <div>
-                        <p className="text-2xl font-bold">{analytics.summary.average_rating.toFixed(1)}</p>
+                        <p className="text-2xl font-bold">{(analytics.summary.average_rating || 0).toFixed(1)}</p>
                         <p className="text-sm text-neutral-600">{t('feedback.avgRating', 'Average Rating')}</p>
                       </div>
                     </div>