Revise cashback processing and payment status logic

[kakun45]

• Updated cashback processing conditions for deposits and clarified payment status checks.
• add a .gitignore file with all [pycache] folders that shouldn't be committed to public
• Consider a disclaimer: "This is NOT how a banking system works, remove all your assumptions of a real world bank, because they will inevitably confuse and lead you onto a wrong path in your implementation"