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/pages/EventChatPage.jsx

@@ -2,15 +2,16 @@ import { useState, useRef, useEffect } from 'react';
 import { useParams, useNavigate } from 'react-router-dom';
 import Layout from '../components/layout/Layout';
 import { useAuth } from '../contexts/AuthContext';
-import { mockEvents } from '../mocks/events';
 import { Send, UserPlus, Loader2 } from 'lucide-react';
 import { connectSocket, disconnectSocket, getSocket } from '../services/socket';
 import { eventsAPI } from '../services/api';
 
 const EventChatPage = () => {
-  const { eventId } = useParams();
+  const { slug } = useParams();
   const { user } = useAuth();
   const navigate = useNavigate();
+  const [event, setEvent] = useState(null);
+  const [loading, setLoading] = useState(true);
   const [messages, setMessages] = useState([]);
   const [newMessage, setNewMessage] = useState('');
   const [activeUsers, setActiveUsers] = useState([]);
@@ -20,17 +21,35 @@ const EventChatPage = () => {
   const messagesEndRef = useRef(null);
   const messagesContainerRef = useRef(null);
 
-  const event = mockEvents.find(e => e.id === parseInt(eventId));
-
   const scrollToBottom = () => {
     messagesEndRef.current?.scrollIntoView({ behavior: 'smooth' });
   };
 
+  // Fetch event data
+  useEffect(() => {
+    const fetchEvent = async () => {
+      try {
+        setLoading(true);
+        const data = await eventsAPI.getBySlug(slug);
+        setEvent(data);
+      } catch (err) {
+        console.error('Error loading event:', err);
+        setEvent(null);
+      } finally {
+        setLoading(false);
+      }
+    };
+
+    fetchEvent();
+  }, [slug]);
+
   useEffect(() => {
     scrollToBottom();
   }, [messages]);
 
   useEffect(() => {
+    if (!event) return;
+
     // Connect to Socket.IO
     const socket = connectSocket();
 
@@ -43,7 +62,7 @@ const EventChatPage = () => {
     socket.on('connect', () => {
       setIsConnected(true);
       // Join event room
-      socket.emit('join_event_room', { eventId: parseInt(eventId) });
+      socket.emit('join_event_room', { slug });
     });
 
     socket.on('disconnect', () => {
@@ -84,7 +103,7 @@ const EventChatPage = () => {
 
     // Cleanup
     return () => {
-      socket.emit('leave_event_room', { eventId: parseInt(eventId) });
+      socket.emit('leave_event_room');
       socket.off('connect');
       socket.off('disconnect');
       socket.off('message_history');
@@ -93,7 +112,7 @@ const EventChatPage = () => {
       socket.off('user_joined');
       socket.off('user_left');
     };
-  }, [eventId, user.id]);
+  }, [event, slug, user.id]);
 
   const handleSendMessage = (e) => {
     e.preventDefault();
@@ -107,7 +126,6 @@ const EventChatPage = () => {
 
     // Send message via Socket.IO
     socket.emit('send_event_message', {
-      eventId: parseInt(eventId),
       content: newMessage,
     });
 
@@ -120,7 +138,7 @@ const EventChatPage = () => {
     setLoadingOlder(true);
     try {
       const oldestMessageId = messages[0].id;
-      const response = await eventsAPI.getMessages(eventId, oldestMessageId, 20);
+      const response = await eventsAPI.getMessages(slug, oldestMessageId, 20);
 
       if (response.data.length > 0) {
         // Save current scroll position
@@ -170,10 +188,27 @@ const EventChatPage = () => {
     return () => container.removeEventListener('scroll', handleScroll);
   }, [loadingOlder, hasMore, messages]);
 
+  if (loading) {
+    return (
+      <Layout>
+        <div className="max-w-4xl mx-auto flex items-center justify-center min-h-[400px]">
+          <div className="flex flex-col items-center gap-3">
+            <Loader2 className="w-12 h-12 animate-spin text-primary-600" />
+            <p className="text-gray-600">Loading event...</p>
+          </div>
+        </div>
+      </Layout>
+    );
+  }
+
   if (!event) {
     return (
       <Layout>
-        <div className="text-center">Event not found</div>
+        <div className="max-w-4xl mx-auto">
+          <div className="bg-red-50 border border-red-200 rounded-md p-4 text-red-700">
+            Event not found
+          </div>
+        </div>
       </Layout>
     );
   }