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.

frontend/src/services/api.js

@@ -186,17 +186,17 @@ export const eventsAPI = {
     return data.data;
   },
 
-  async getById(id) {
-    const data = await fetchAPI(`/events/${id}`);
+  async getBySlug(slug) {
+    const data = await fetchAPI(`/events/${slug}`);
     return data.data;
   },
 
-  async getMessages(eventId, before = null, limit = 20) {
+  async getMessages(slug, before = null, limit = 20) {
     const params = new URLSearchParams({ limit: limit.toString() });
     if (before) {
       params.append('before', before.toString());
     }
-    const data = await fetchAPI(`/events/${eventId}/messages?${params}`);
+    const data = await fetchAPI(`/events/${slug}/messages?${params}`);
     return data;
   },
 };