fix: improve URL slug generation for special characters

- Replace all non-alphanumeric characters with single dash
- Collapse multiple consecutive dashes into single dash
- Remove leading and trailing dashes
- Fixes issue where "Petra & Peter" became "petra---peter"
- Now generates cleaner URLs like "petra-peter" instead

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

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

backend/src/routes/adminEvents.js

@@ -66,7 +66,12 @@ router.post('/', adminAuth, [
     }
     
     // Generate unique slug
-    const baseSlug = `${event_type}-${event_name.toLowerCase().replace(/[^a-z0-9]/g, '-')}-${event_date}`;
+    const processedEventName = event_name
+      .toLowerCase()
+      .replace(/[^a-z0-9]/g, '-') // Replace non-alphanumeric with dash
+      .replace(/-+/g, '-')         // Replace multiple dashes with single dash
+      .replace(/^-|-$/g, '');      // Remove leading/trailing dashes
+    const baseSlug = `${event_type}-${processedEventName}-${event_date}`;
     let slug = baseSlug;
     let counter = 1;