feat(chat): add country flags and competitor numbers with normalized data architecture

Implemented display of country flags and competitor numbers in event chat messages:
- Country flags displayed as emoji (🇸🇪, 🇵🇱, etc.) with proper emoji font support
- Competitor numbers shown in #123 format next to usernames
- Normalized data architecture with user and participant caches on frontend
- User data (username, avatar, country) and participant data (competitorNumber) cached separately
- Messages store only core data (id, content, userId, createdAt)
- Prevents data inconsistency when users update profile information
- Fixed duplicate message keys React warning with deduplication logic
- Backend sends nested user/participant objects for cache population
- Auto-updates across all messages when user changes avatar or country

Backend changes:
- Socket.IO event_message and message_history include nested user/participant data
- API /events/:slug/messages endpoint restructured with same nested format
- Batch lookup of competitor numbers for efficiency

Frontend changes:
- useEventChat hook maintains userCache and participantCache
- ChatMessage component accepts separate user/participant props
- ChatMessageList performs cache lookups during render
- Emoji font family support for cross-platform flag rendering

backend/src/routes/events.js

@@ -159,6 +159,7 @@ router.get('/:slug/messages', authenticate, async (req, res, next) => {
             id: true,
             username: true,
             avatar: true,
+            country: true,
           },
         },
       },
@@ -166,6 +167,24 @@ router.get('/:slug/messages', authenticate, async (req, res, next) => {
       take: parseInt(limit),
     });
 
+    // Get competitor numbers for all users in this event
+    const userIds = [...new Set(messages.map(msg => msg.user.id))];
+    const eventParticipants = await prisma.eventParticipant.findMany({
+      where: {
+        eventId: event.id,
+        userId: { in: userIds },
+      },
+      select: {
+        userId: true,
+        competitorNumber: true,
+      },
+    });
+
+    // Create a map of userId to competitorNumber
+    const competitorNumberMap = new Map(
+      eventParticipants.map(ep => [ep.userId, ep.competitorNumber])
+    );
+
     // Return in chronological order (oldest first)
     res.json({
       success: true,
@@ -173,11 +192,20 @@ router.get('/:slug/messages', authenticate, async (req, res, next) => {
         id: msg.id,
         roomId: msg.roomId,
         userId: msg.user.id,
-        username: msg.user.username,
-        avatar: msg.user.avatar,
         content: msg.content,
         type: msg.type,
         createdAt: msg.createdAt,
+        // Nested user data for caching
+        user: {
+          id: msg.user.id,
+          username: msg.user.username,
+          avatar: msg.user.avatar,
+          country: msg.user.country,
+        },
+        // Nested participant data for caching
+        participant: {
+          competitorNumber: competitorNumberMap.get(msg.user.id),
+        },
       })),
       hasMore: messages.length === parseInt(limit),
     });