feat: SEO backend data (1/4)

features/activity-log-producers.feature

@@ -84,7 +84,8 @@ Feature: Every real action of the back-office is recorded in the activity log fr
     Scenario: A title change is recorded as a text diff when a title edit exists
       Given a use case that changes an article's title from "Spring release" to "Spring release 2"
       Then its event has action "article.edit" and changes [{"field": "title", "kind": "text", "before": "Spring release", "after": "Spring release 2"}]
-      And only the fields title, online and body can ever appear in an article event's changes
+      And only the fields title, online, body and, since features/seo_data_backend.feature, site, authors and the SEO fields can ever appear in an article event's changes
+      And SEO free-text fields (excerpt, seo_title, seo_description, social_title, social_description, cover_image_alt) are recorded as a size delta in characters with kind "body" and never as text, while short values (slug, canonical_url, noindex, cover_image, social_image) are recorded as text before and after
 
     Scenario: Publishing is recorded with the publish state
       Given the article 8 "Spring release" is offline