🩹(backend) fix Django Admin UserAdmin page

*Broken Identity string representation
Resolving a format error in the Identity string representation caused by
potential None values in the email field. This issue was discovered when
attempting to access the User details page in the Django Admin

*Broken User creation form
The replacement of the User's username with an email led to errors in
the UserAdmin class. The base class used the 'username' field in the
'add_fieldsets' attribute. This problem was discovered while attempting to
create a new user in the Django Admin.

src/backend/core/admin.py

@@ -82,6 +82,15 @@ class UserAdmin(auth_admin.UserAdmin):
         ),
         (_("Important dates"), {"fields": ("created_at", "updated_at")}),
     )
+    add_fieldsets = (
+        (
+            None,
+            {
+                "classes": ("wide",),
+                "fields": ("email", "password1", "password2"),
+            },
+        ),
+    )
     inlines = (IdentityInline, TeamAccessInline)
     list_display = (
         "email",

src/backend/core/models.py

@@ -288,7 +288,8 @@ class Identity(BaseModel):
 
     def __str__(self):
         main_str = "[main]" if self.is_main else ""
-        return f"{self.email:s}{main_str:s}"
+        id_str = self.email or self.sub
+        return f"{id_str:s}{main_str:s}"
 
     def save(self, *args, **kwargs):
         """Ensure users always have one and only one main identity."""