feat: return distinct 'draft' error for draft post detail requests

Previously the API returned 'not_found' for both missing and draft
posts. Now draft posts return a specific 'draft' error code so API
consumers can distinguish between the two cases and show an
appropriate message.

lib/volumen/api_routes.rb

@@ -28,7 +28,8 @@ module Volumen
 
       app.get "/api/volumen/posts/:slug" do
         post = store.find(params["slug"], lang: params["lang"])
-        halt(404, json("error" => "not_found")) if post.nil? || post.draft?
+        halt(404, json("error" => "not_found")) if post.nil?
+        halt(404, json("error" => "draft")) if post.draft?
 
         json(post.detail)
       end