Added task description

Author: vodkar

TASK.md

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+# Backend task for implementation
+
+Create a backend endpoints which implements following functionality:
+
+- Introduce a new entity Wallet and Transaction.
+- Wallet should have fields: id, user_id (foreign key to User), balance (float), currency (string).
+- Available currencies: USD, EUR, RUB.
+- Transaction should have fields: id, wallet_id (foreign key to Wallet), amount (float), type (enum: 'credit', 'debit'), timestamp (datetime), currency (string).
+- Implement endpoint to create a wallet for a user.
+- Implement endpoint to get wallet details including current balance.
+- Implement endpoint to create a transaction (credit or debit) for a wallet.
+
+# Rules for wallet
+
+- A user can have three wallets.
+- Wallet balance should start at 0.0.
+- Arithmetic operations on balance should be precise up to two decimal places.
+
+# Rules for transaction
+
+- For 'credit' transactions, the amount should be added to the wallet balance.
+- For 'debit' transactions, the amount should be subtracted from the wallet balance.
+- Ensure that the wallet balance cannot go negative. If a debit transaction would cause the balance to go negative, the transaction should be rejected with an appropriate error message.
+- Transaction between wallets with different currencies must be converted using a fixed exchange rate (you can hardcode some exchange rates for simplicity) and fees should be applied.