refactor(dto): change seq field type from string to int64

Update the `Seq` field in `WSEventResponse` and its usages across message and websocket handlers to use `int64` instead of `string`. This aligns the data transfer object with the underlying message sequence type and removes unnecessary string conversions.

internal/handler/message_handler.go

@@ -418,7 +418,7 @@ func (h *MessageHandler) HandleSendMessage(c *gin.Context) {
 		Type:           "message",
 		DetailType:     params.DetailType,
 		ConversationID: conversationID,
-		Seq:            strconv.FormatInt(msg.Seq, 10),
+		Seq:            msg.Seq,
 		Segments:       params.Segments,
 		SenderID:       userID,
 	}