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
 }