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 <[email protected]>

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>