Settings option to disable email alerts

### Problem

Defguard sends the "User enrollment completed" email to the administrator who started an enrollment after the user completes it.

This is useful for some deployments, but can create unnecessary email volume in organisations that enroll many users or devices. There is currently no way to disable this notification.

### Requested functionality

Add configurable email-notification preferences for administrators.

Initially, administrators should be able to disable the following notification:

- User enrollment completed

The configuration should apply to future notifications only and must not affect enrollment itself or emails sent to the enrolled user.

### Future scope

Consider extending this mechanism to other administrative email alerts, for example:

- Gateway disconnected or reconnected
- Certificate expiration or expiration failures
- Let's Encrypt certificate refresh failures
- User import blocked

Each notification type may be configurable independently.