Allow users to manage backup retention and backup logs

**Mandatory**
- Keep site backup logs on press irrespective of status / cleanup actions

**Additional**
- Allow users to configure backup retention
- Like keep all backups of 1 year, 2 year (instead of purging based on grandfather-father-son algo)
- Charge customers for the additional backups

The issue names no files, tests, or entry points. Start by locating the existing backup cleanup, log retention, and grandfather-father-son logic, then trace how retention settings and backup storage costs could connect to customer billing; done means logs survive cleanup and status changes, configurable retention periods work, and additional backups are chargeable.

Written by the indexing model from the issue text.