Send security notification when recovery code is used or created by admin

"Where to store recovery codes" is one of these problems there is no
right answer to, so many people store them in a less-than-optimal place.
If that's the reality we live in, this PR adds at least a little
security so one notices when they get used :)

src/pretix/control/views/auth.py

@@ -57,6 +57,7 @@ from django.views.decorators.csrf import csrf_exempt
 from django.views.decorators.http import require_http_methods
 from django.views.generic import TemplateView
 from django_otp import match_token
+from django_otp.plugins.otp_static.models import StaticDevice
 from webauthn.helpers import generate_challenge
 
 from pretix.base.auth import get_auth_backends
@@ -538,6 +539,10 @@ class Login2FAView(TemplateView):
                     break
         else:
             valid = match_token(self.user, token)
+            if isinstance(valid, StaticDevice):
+                self.user.send_security_notice([
+                    _("A recovery code for two-factor authentification was used to log in.")
+                ])
 
         if valid:
             logger.info(f"Backend login successful for user {self.user.pk} with 2FA.")

src/pretix/control/views/users.py

@@ -165,6 +165,9 @@ class UserEmergencyTokenView(AdministratorPermissionRequiredMixin, RecentAuthent
         d, __ = StaticDevice.objects.get_or_create(user=self.object, name='emergency')
         token = d.token_set.create(token=get_random_string(length=12, allowed_chars='1234567890'))
         self.object.log_action('pretix.user.settings.2fa.emergency', user=self.request.user)
+        self.object.send_security_notice([
+            _('A two-factor emergency code has been generated by a system administrator.')
+        ])
 
         messages.success(request, _(
             'The emergency token for this user is "{token}". It can only be used once. Please make sure to transmit '

src/tests/control/test_auth.py

@@ -42,8 +42,10 @@ from django.contrib.auth.tokens import (
 )
 from django.core import mail as djmail
 from django.test import RequestFactory, TestCase, override_settings
+from django.utils.crypto import get_random_string
 from django.utils.timezone import now
 from django_otp.oath import TOTP
+from django_otp.plugins.otp_static.models import StaticDevice
 from django_otp.plugins.otp_totp.models import TOTPDevice
 from webauthn.authentication.verify_authentication_response import (
     VerifiedAuthentication,
@@ -492,6 +494,20 @@ class Login2FAFormTest(TestCase):
 
         m.undo()
 
+    def test_recovery_code_valid(self):
+        djmail.outbox = []
+        d, __ = StaticDevice.objects.get_or_create(user=self.user, name='emergency')
+        token = d.token_set.create(token=get_random_string(length=12, allowed_chars='1234567890'))
+
+        response = self.client.get('/control/login/2fa')
+        assert 'token' in response.content.decode()
+        response = self.client.post('/control/login/2fa', {
+            'token': token.token,
+        })
+        self.assertEqual(response.status_code, 302)
+        self.assertIn('/control/', response['Location'])
+        assert "recovery code" in djmail.outbox[0].body
+
 
 class FakeRedis(object):
     def get_redis_connection(self, connection_string):