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

frontend/src/pages/EventChatPage.jsx

@@ -38,6 +38,8 @@ const EventChatPage = () => {
     isConnected,
     loadingOlder,
     hasMore,
+    userCache,
+    participantCache,
     sendMessage: handleSendMessage,
     loadOlderMessages
   } = useEventChat(slug, user?.id, event, messagesContainerRef);
@@ -478,6 +480,8 @@ const EventChatPage = () => {
                 <div className="flex-1 flex flex-col">
                   <ChatMessageList
                     messages={messages}
+                    userCache={userCache}
+                    participantCache={participantCache}
                     currentUserId={user.id}
                     messagesEndRef={messagesEndRef}
                     messagesContainerRef={messagesContainerRef}