feat: public API, SEO and existing surfaces

api/pkg/apperr/messages.go

@@ -171,3 +171,8 @@ const SlugConflictErrorMessage = "slug_conflict"
 const ConflictingArticlesHeader = "X-Conflicting-Articles"
 const StoryMoveRequiredErrorMessage = "story_move_required"
 const StoryBusyErrorMessage = "story_busy"
+
+const ArticleNotFoundCode = "article_not_found"
+const LanguageNotAvailableCode = "language_not_available"
+const SiteNotFoundCode = "site_not_found"
+const InvalidLanguageCode = "invalid_language"