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/ws_handler.go

@@ -6,7 +6,6 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
-	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
@@ -459,7 +458,7 @@ func (h *WSHandler) handleChat(ctx context.Context, client *ws.Client, payload j
 		Type:           "message",
 		DetailType:     detailType,
 		ConversationID: req.ConversationID,
-		Seq:            strconv.FormatInt(message.Seq, 10),
+		Seq:            message.Seq,
 		Segments:       req.Segments,
 		SenderID:       client.UserID,
 	}