Alert Rule Backup and Restore / Version History

### Problem Statement

When alert rules are accidentally deleted or edited, there is no way to recover them. There is no backup, versioning, or soft-delete mechanism for alert rule configurations. Recovery currently requires an ad-hoc engineering effort to pull data from the database before it's permanently removed.

### Solution Brainstorm

- Soft-delete with a retention window (e.g. 30 days) so deleted alert rules can be restored
- Version history for alert rules
- A "trash" or "recently deleted" section in the Alerts UI where deleted rules can be recovered

### Product Area

Alerts

Start by mapping the alert-rule persistence area and the Alerts UI; the issue names no files, tests, or entry points. Compare the proposed soft-delete, retention, version-history, and recovery options with existing alert behavior. Done requires an agreed design and defined implementation scope, which this issue does not yet provide.

Written by the indexing model from the issue text.