🚸(oidc) ignore case when fallback on email

Some identity providers might change the case, but in our products we
don't consider case variation to be consider as different email
addresses.

Next step would be to normalize the DB value of email to be lower-case.

src/backend/core/models.py

@@ -118,11 +118,11 @@ class UserManager(auth_models.UserManager):
 
             if settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION:
                 try:
-                    return self.get(email=email)
+                    return self.get(email__iexact=email)
                 except self.model.DoesNotExist:
                     pass
             elif (
-                self.filter(email=email).exists()
+                self.filter(email__iexact=email).exists()
                 and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
             ):
                 raise DuplicateEmailError(
@@ -1905,7 +1905,7 @@ class Invitation(BaseModel):
 
         # Check if an identity already exists for the provided email
         if (
-            User.objects.filter(email=self.email).exists()
+            User.objects.filter(email__iexact=self.email).exists()
             and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
         ):
             raise ValidationError(