[UX] Toggle to enable/disable Duo logging

Problem

Currently, AI logging is controlled by a feature flag (expanded_ai_logging), which is not user-friendly or suitable for long-term use. We need to provide a user-friendly way to enable/disable AI logging for self-hosted GitLab instances. A toggle in the admin area would provide a better experience.

Out of Scope

Setting log retention periods (separate issue: #487374)

Proposed Design