Propose a better way of testing email notifications

### Description

Since we've migrated away from a shared email channel and now just use `send_and_forget()` it's not possible to test in our integration tests if correct emails are being sent in specific API scenarios.

Right now we do have some tests marked with:
```
#[ignore = "requires SMTP server"]
#[sqlx::test]
```
But this is not reasonable to run in CI.

Example test scenario I'd like to achieve:
- enable LDAP remote enrollment + automatic invites
- sync user from LDAP
- verify that the enrollment invite is sent