🐛(backend) prevent duplicate pending users on concurrent requests

Fix a race condition in the external viewset where concurrent requests
could create multiple pending users with the same email address.

No database constraint enforced email uniqueness for pending users,
allowing duplicates to be created under load.

This caused issues during user reconciliation, which expects a single
matching pending user and raises a warning when multiple records are
found.

Add a narrowly scoped migration to enforce the uniqueness constraint
and address the identified issue.

src/backend/core/tests/authentication/test_backends.py

@@ -346,8 +346,11 @@ def test_authentication_getter_existing_user_change_fields(
 
     # One and only one additional update query when a field has changed
     # Note: .save() triggers uniqueness validation queries for unique fields,
-    # adding extra SELECT queries before the UPDATE (e.g., checking unique=True on 'sub')
-    with django_assert_num_queries(3):
+    # adding extra SELECT queries before the UPDATE:
+    #   - unique=True on 'sub'
+    #   - unique=True on 'admin_email'
+    #   - partial unique index 'unique_email_when_sub_is_null'
+    with django_assert_num_queries(5):
         authenticated_user = klass.get_or_create_user(
             access_token="test-token", id_token=None, payload=None
         )