Add the health check of the backup when backup finished

## Feature Request

**Is your feature request related to a problem? Please describe:**

when GC safepoint not advanced for a long time. backup will generate too many small SST files. which will makes restore slowly.
**Describe the feature you'd like:**

Add a health check of backup. to let backup user know the potential performance issue when backup finished.

**Describe alternatives you've considered:**

1. report the number of small SST Files in backup.

**Teachability, Documentation, Adoption, Migration Strategy:**

The issue does not name files, tests, or a backup entry point. Start by locating the backup completion flow and the code that creates or reports SST files; clarify the health-check criteria and expected user-facing report before implementation. Done should include a defined check that identifies the small-SST-file condition and reports it when backup finishes.

Written by the indexing model from the issue text.