Performance: domains.map(&:destroy) instead of destroy_all in SetupController

## Description

In `app/controllers/setup_controller.rb:63, 78`:

```ruby
@account.domains.map(&:destroy)
```

This loads each domain and destroys one-by-one (N+1 deletes). 

## Recommendation

Use `@account.domains.destroy_all` for a single operation.

## Severity

Low