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.

test/server_test.rb

@@ -104,10 +104,10 @@ class ServerTest < Minitest::Test
     assert_includes body, "<language>en</language>"
   end
 
-  def test_draft_post_detail_is_not_found
+  def test_draft_post_detail_returns_draft_error
     get "/api/volumen/posts/draft"
     assert_equal 404, last_response.status
-    assert_equal "not_found", JSON.parse(last_response.body)["error"]
+    assert_equal "draft", JSON.parse(last_response.body)["error"]
   end
 
   def test_sitemap_endpoint