Add email notifications for incomplete 2FA logins

An incomplete 2FA login is one where the correct master password was provided,
but the 2FA token or action required to complete the login was not provided
within the configured time limit. This potentially indicates that the user's
master password has been compromised, but the login was blocked by 2FA.

Be aware that the 2FA step can usually still be completed after the email
notification has already been sent out, which could be confusing. Therefore,
the incomplete 2FA time limit should be long enough that this situation would
be unlikely. This feature can also be disabled entirely if desired.

src/main.rs

@@ -345,6 +345,14 @@ fn schedule_jobs(pool: db::DbPool) {
                 }));
             }
 
+            // Send email notifications about incomplete 2FA logins, which potentially
+            // indicates that a user's master password has been compromised.
+            if !CONFIG.incomplete_2fa_schedule().is_empty() {
+                sched.add(Job::new(CONFIG.incomplete_2fa_schedule().parse().unwrap(), || {
+                    api::send_incomplete_2fa_notifications(pool.clone());
+                }));
+            }
+
             // Grant emergency access requests that have met the required wait time.
             // This job should run before the emergency access reminders job to avoid
             // sending reminders for requests that are about to be granted anyway.