Domain-model.md

+The problem domain focuses on the user, which is the center of the diagram.
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+Each user can have zero to several badges, challenges and goals. 
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+Each user has to be linked to two different accounts (saving and spending).
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+![image](uploads/9ef852da35a5194aa2626cd2728d7d66/image.png)
+*version 0.1 - 16th April 2024*