feat: add event slugs to prevent ID enumeration attacks

Replace sequential event IDs in URLs with unique alphanumeric slugs to prevent enumeration attacks. Event URLs now use format /events/{slug}/chat instead of /events/{id}/chat.

Backend changes:
- Add slug field (VARCHAR 50, unique) to Event model
- Create migration with auto-generated 12-char MD5-based slugs for existing events
- Update GET /api/events/:slug endpoint (changed from :id)
- Update GET /api/events/:slug/messages endpoint (changed from :eventId)
- Modify Socket.IO join_event_room to accept slug parameter
- Update send_event_message to use stored event context instead of passing eventId

Frontend changes:
- Update eventsAPI.getBySlug() method (changed from getById)
- Update eventsAPI.getMessages() to use slug parameter
- Change route from /events/:eventId/chat to /events/:slug/chat
- Update EventsPage to navigate using event.slug
- Update EventChatPage to fetch event data via slug and use slug in socket events

Security impact: Prevents attackers from discovering all events by iterating sequential IDs.

backend/prisma/schema.prisma

@@ -61,6 +61,7 @@ model User {
 // Events table (dance events from worldsdc.com)
 model Event {
   id                Int       @id @default(autoincrement())
+  slug              String    @unique @db.VarChar(50)
   name              String    @db.VarChar(255)
   location          String    @db.VarChar(255)
   startDate         DateTime  @map("start_date") @db.Date