Add human-readable event URL slugs with legacy redirect support.

Store unique slugs on events, backfill existing records, redirect old UUID and alias URLs to canonical slug pages, and expose slug editing plus alias management in the admin event modal.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/(public)/book/[eventId]/page.tsx

@@ -166,7 +166,7 @@ export default function BookingPage() {
           const soldOut = bookedCount >= capacity;
           if (soldOut) {
             toast.error(t('events.details.soldOut'));
-            router.push(`/events/${eventRes.event.id}`);
+            router.push(`/events/${eventRes.event.slug}`);
             return;
           }
 
@@ -1049,7 +1049,7 @@ export default function BookingPage() {
     <div className="section-padding bg-secondary-gray min-h-screen">
       <div className="container-page max-w-2xl">
         <Link 
-          href={`/events/${event.id}`}
+          href={`/events/${event.slug}`}
           className="inline-flex items-center gap-2 text-gray-600 hover:text-primary-dark mb-6"
         >
           <ArrowLeftIcon className="w-4 h-4" />