feat(API): media library

features/image_upload_rules.feature

@@ -17,8 +17,8 @@ Feature: Allowed image types and the size limit are a domain rule
       the matching sentinel and calling the repository not at all when either check fails
     And ImageRepository.Create no longer rejects an unrecognized content type itself; it
       only maps an already-allowed content type to a file extension
-    And no image HTTP endpoint is mounted in api/router.go as of this slice, so this
-      behavior is observable only through CreateImageUseCase
+    And the HTTP mapping of these sentinels is 413, 415 and 400, specified in
+      features/media_library.feature
 
   Scenario Outline: CreateImage accepts every allowed content type within the size limit
     Given an empty image repository fake