feat(dashboard): add unread count for match chats

Track unread messages in match chats and display count badge:
- Schema: Add user1LastReadAt/user2LastReadAt to Match model
- Backend: Calculate unreadCount in dashboard API
- Socket: Update lastReadAt when user joins match room
- Frontend: Display red badge with unread count on match avatar

frontend/src/pages/DashboardPage.jsx

@@ -356,7 +356,7 @@ const EventCard = ({ event }) => {
 // Match Card Component
 const MatchCard = ({ match }) => {
   const navigate = useNavigate();
-  const { partner, event, videoExchange, ratings } = match;
+  const { partner, event, videoExchange, ratings, unreadCount } = match;
 
   // Can rate when video exchange is complete and user hasn't rated yet
   const canRate = videoExchange?.sentByMe && videoExchange?.receivedFromPartner && !ratings?.ratedByMe;
@@ -365,12 +365,17 @@ const MatchCard = ({ match }) => {
     <div className="bg-white rounded-lg shadow-sm border border-gray-200 p-4 hover:shadow-md transition-shadow">
       <div className="flex items-start gap-4">
         {/* Avatar */}
-        <Link to={`/${partner.username}`} className="flex-shrink-0">
+        <Link to={`/${partner.username}`} className="flex-shrink-0 relative">
           <img
             src={partner.avatar || `https://api.dicebear.com/7.x/avataaars/svg?seed=${partner.username}`}
             alt={partner.username}
             className="w-12 h-12 rounded-full hover:ring-2 hover:ring-primary-500 transition-all"
           />
+          {unreadCount > 0 && (
+            <span className="absolute -top-1 -right-1 bg-red-500 text-white text-xs rounded-full h-5 w-5 flex items-center justify-center font-semibold">
+              {unreadCount > 9 ? '9+' : unreadCount}
+            </span>
+          )}
         </Link>
 
         {/* Content */}