We should have additional field in order, repaid_amount

[Vovke]

We need repaid_amount field as part of the order structure in order to easily understand how much was repaid already. Main case is when the order is repaid in chunks, we can calculate repaid amount using transactions, but that means we want frontend to calculate it for us which is potential for mistakes.

There are 2 options for implementation,

• we update repaid_amount with each transaction happening and store it in the database among other order fields
• we calculate repaid_amount based on the sum of transactions

[Vovke]

based on #77 we update repaid_amount with each order's incoming transaction.

ensure that this change reflected in:

• https://github.com/Alzymologist/kalatori-api
• https://github.com/Alzymologist/Kalatori-backend/blob/main/docs/DATABASE.md