Use background jobs for slow operations

Move slow operations to background jobs:
- Email sending
- File processing
- External API calls
- Report generation
- Data imports/exports

Use SolidQueue (Rails 8) or Sidekiq. Keep web requests under 200ms.

```ruby
# Good: Background job
SendWelcomeEmailJob.perform_later(user)

# Bad: Synchronous in controller
UserMailer.welcome(user).deliver_now
```