feat: SEO backend data (1/4)

features/sites_backend.feature

@@ -562,9 +562,10 @@ Feature: Sites backend, the owner manages sites and every article belongs to exa
       Given the purge of a group fails midway
       Then every site default_social_image_id refers to an image that still exists
 
-    Scenario: The usage lookup array shape is unchanged
+    Scenario: The usage lookup stays an array and later gains additive keys
       When a verified account looks up the usage of an image
-      Then the response keeps the same array shape as before this slice
+      Then the response is a JSON array whose article items keep id, title and published
+      And features/seo_data_backend.feature adds type and fields to every item and lists authors and sites, without removing any key
 
   Rule: Every site write is recorded in the activity log