sync: phase 1 backend + frontend from server

internal/gateway/hub.go

@@ -281,6 +281,46 @@ func (h *Hub) BroadcastToServer(serverID string, event Event) {
 	}
 }
 
+// BroadcastToConversation sends an event to all clients who are members of a
+// given direct-message conversation.
+func (h *Hub) BroadcastToConversation(convID string, event Event) {
+	data, err := json.Marshal(event)
+	if err != nil {
+		h.logger.Error("failed to marshal event", "type", event.Type, "error", err)
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	rows, err := h.db.QueryContext(ctx, `SELECT user_id FROM conversation_members WHERE conversation_id = $1`, convID)
+	if err != nil {
+		h.logger.Error("failed to load conversation members", "conversation_id", convID, "error", err)
+		return
+	}
+	defer rows.Close()
+
+	members := make(map[string]struct{})
+	for rows.Next() {
+		var uid string
+		if err := rows.Scan(&uid); err == nil {
+			members[uid] = struct{}{}
+		}
+	}
+
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	for client := range h.clients {
+		if _, ok := members[client.UserID]; ok {
+			select {
+			case client.send <- data:
+			default:
+				go func(c *Client) { h.unregister <- c }(client)
+			}
+		}
+	}
+}
+
 // ServerIDForChannel looks up the server_id that owns the given channel.
 // Used by handlers to route events to the correct server scope.
 func (h *Hub) ServerIDForChannel(ctx context.Context, channelID string) (string, error) {