refactor: extract rate formatting logic to RateService and add tests

.env.example

@@ -55,6 +55,8 @@ MAIL_USERNAME=null
 MAIL_PASSWORD=null
 MAIL_FROM_ADDRESS="hello@example.com"
 MAIL_FROM_NAME="${APP_NAME}"
+MAIL_ADMIN_ADDRESS="watheq.alshowaiter@gmail.com"
+MAIL_ADMIN_NAME="Watheq Alshowaiter"
 
 AWS_ACCESS_KEY_ID=
 AWS_SECRET_ACCESS_KEY=