refactor: Change balance type to bigdecimal for better precision

backend/requests/user.http

@@ -16,3 +16,12 @@ Authorization: Bearer {{token}}
   "username": "john.doe"
 }
 
+### Deposit
+POST http://localhost:8080/deposit/checkout
+Content-Type: application/json
+Origin: http://localhost:8080
+Authorization: Bearer {{token}}
+
+{
+  "amount": 60.12
+}