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/admin/events/[id]/page.tsx

@@ -594,7 +594,7 @@ export default function AdminEventDetailPage() {
             {showStats ? <EyeSlashIcon className="w-4 h-4 mr-1.5" /> : <EyeIcon className="w-4 h-4 mr-1.5" />}
             {showStats ? 'Hide Stats' : 'Show Stats'}
           </Button>
-          <Link href={`/events/${event.id}`} target="_blank">
+          <Link href={`/events/${event.slug}`} target="_blank">
             <Button variant="outline" size="sm">
               <EyeIcon className="w-4 h-4 mr-1.5" />
               View Public
@@ -618,7 +618,7 @@ export default function AdminEventDetailPage() {
               </button>
             }
           >
-            <DropdownItem onClick={() => { window.open(`/events/${event.id}`, '_blank'); setMobileHeaderMenuOpen(false); }}>
+            <DropdownItem onClick={() => { window.open(`/events/${event.slug}`, '_blank'); setMobileHeaderMenuOpen(false); }}>
               <EyeIcon className="w-4 h-4 mr-2" /> View Public
             </DropdownItem>
             <DropdownItem onClick={() => { router.push(`/admin/events?edit=${event.id}`); setMobileHeaderMenuOpen(false); }}>