The solution is no concurrency safe — transaction is not atomic
me — there is no explicit requirement and actually basic
transaction (add) is atomic. There is no race conditions. Atomicity of
_transfer_ is required if we don't want to loose data — but here there
is no such problem since we don;t store anything between sessions.

Insufficient testing coverage
nothing to say. It's 6hrs test. Not 40hrs

If we transfer negative amount money, author don’t throw exceptions, it takes abs value of this amount and transfer positive amount of money.
transferring negative amount is something which is odd. So it's a
convention. It either has to be documented or exception should be
thrown. So I partially agree.

Author don't know how to catch exceptions in junit
minor. Nothing to add.

Procedure of transferring money is not clear and difficult to support
no comment.

Poor documentation — there is no specifications of endpoints, how to build a project
KL — there is a README. And there is a build.gradle. It's kind of
misunderstanding what we expect from 6hrs test.