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>

backend/src/routes/adminEvents.js

@@ -53,7 +53,7 @@ router.post('/', adminAuth, [
     } = req.body;
     
     // Validate password strength
-    const passwordValidation = validatePasswordInContext(password, 'gallery', {
+    const passwordValidation = await validatePasswordInContext(password, 'gallery', {
       eventName: event_name
     });