add synced flag to the scen table.

The puzzle `45-abfeb6fc` from #45 has to be resolved:

https://github.com/ammaratef45/Attendance/blob/820c42da11108754c8e553a91a829d24faa191b8/attendance/lib/backend/scan.dart#L101-L101

The puzzle was created by Ammar Atef on 07-May-19.

If you have any technical questions, don't ask me, submit new tickets instead. The task will be \"done\" when the problem is fixed and the text of the puzzle is _removed_ from the source code. Here is more about [PDD](http://www.yegor256.com/2009/03/04/pdd.html) and [about me](http://www.yegor256.com/2017/04/05/pdd-in-action.html).