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/sitemap.ts

@@ -5,6 +5,7 @@ const apiUrl = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001';
 
 interface SitemapEvent {
   id: string;
+  slug: string;
   status: string;
   startDatetime: string;
   updatedAt: string;
@@ -100,7 +101,7 @@ export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
   const eventPages: MetadataRoute.Sitemap = events.map((event) => {
     const isUpcoming = new Date(event.startDatetime) > now;
     return {
-      url: `${siteUrl}/events/${event.id}`,
+      url: `${siteUrl}/events/${event.slug}`,
       lastModified: new Date(event.updatedAt),
       changeFrequency: isUpcoming ? ('weekly' as const) : ('monthly' as const),
       priority: isUpcoming ? 0.8 : 0.5,