## Description
The RemoveOrphanConversationsService filters orphan conversations by a
time window before deleting them. Previously it used created_at, which
could miss old conversations that still had recent activity.
Switching to last_activity_at ensures the cleanup window reflects actual
conversation activity rather than creation time.
## Type of change
Please delete options that are not relevant.
- [ ] Bug fix (non-breaking change which fixes an issue)
## How Has This Been Tested?
- By running Rake task
- Run the job from console
## Checklist:
- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules
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