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/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,
 	}