Board ownership transfer to non-existent user

[DorraJaouad]

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Describe the bug
The API endpoint responsible for transferring board ownership allows a user to specify a new owner via the newOwner parameter. However, the backend service does not validate whether the specified user actually exists in the system.

To Reproduce
Steps to reproduce the behavior:

1. Create a new Deck board.
2. Transfer ownership with /apps/deck/boards/26/transferOwner using wrong new owner.
3. The requests is successful

Expected behavior
It returns an error Invalid request