refactor code, add more flexibility for filtering transactions by periods of time

src/main/java/com/faf223/expensetrackerfaf/service/ExpenseService.java

@@ -12,6 +12,7 @@ import org.springframework.security.core.Authentication;
 import org.springframework.security.core.context.SecurityContextHolder;
 import org.springframework.security.core.userdetails.UserDetails;
 import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
 
 import java.time.LocalDate;
 import java.time.Month;