cold backup计时bug

## Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
冷备策略设置中，当间隔时间<1天时，会无法正常工作。现象为上一个冷备结束后立即进行下一次冷备，但不会一直下去，大概四五次就会长时间不进行下一次备份（不确定是不是再也不触发）。

2. What did you expect to see?

3. What did you see instead?

4. What version of Pegasus are you using?

Start by reproducing the cold-backup configuration with an interval under one day and observe scheduling after several completed backups. Trace the cold-backup scheduling path to determine why backups run immediately several times and then stop for a long period. Done means repeated backups continue at the configured interval.

Written by the indexing model from the issue text.