Fix missing date_of_birth in admin user detail API response

UserDetailResponse was built from UserListItem (which excludes
date_of_birth), so the field always returned null. Explicitly
pass user.date_of_birth to the response constructor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routers/admin.py

@@ -180,6 +180,7 @@ async def get_user(
         **UserListItem.model_validate(user).model_dump(exclude={"active_sessions"}),
         active_sessions=active_sessions,
         preferred_name=preferred_name,
+        date_of_birth=user.date_of_birth,
     )