code refactor and add unit tests

pkg/cron/cron_jobs.go

@@ -1,15 +1,16 @@
 package cron
 
 import (
-	"time"
-
 	"github.com/mayswind/ezbookkeeping/pkg/services"
 )
 
+// RemoveExpiredTokensJob represents the cron job which periodically remove expired user tokens from the database
 var RemoveExpiredTokensJob = &CronJob{
 	Name:        "RemoveExpiredTokens",
 	Description: "Periodically remove expired user tokens from the database.",
-	Interval:    24 * time.Hour,
+	Period: CronJobFixedHourPeriod{
+		Hour: 0,
+	},
 	Run: func() error {
 		return services.Tokens.DeleteAllExpiredTokens(nil)
 	},