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/dto/dto.go

@@ -745,7 +745,7 @@ type WSEventResponse struct {
 	Type           string                `json:"type"`            // 事件类型 (message, notification, system等)
 	DetailType     string                `json:"detail_type"`     // 详细类型 (private, group, like, comment等)
 	ConversationID string                `json:"conversation_id"` // 会话ID
-	Seq            string                `json:"seq"`             // 消息序列号
+	Seq            int64                 `json:"seq"`              // 消息序列号
 	Segments       model.MessageSegments `json:"segments"`        // 消息段数组
 	SenderID       string                `json:"sender_id"`       // 发送者用户ID
 }

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,
 	}

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,
 	}

internal/service/push_service.go

@@ -242,7 +242,7 @@ func (s *pushServiceImpl) pushViaWebSocket(ctx context.Context, userID string, m
 		Type:           "message",
 		DetailType:     detailType,
 		ConversationID: message.ConversationID,
-		Seq:            fmt.Sprintf("%d", message.Seq),
+		Seq:            message.Seq,
 		Segments:       segments,
 		SenderID:       message.SenderID,
 	}