feat: expose closed boolean in card JSON API

Adds `closed` field to card JSON response, allowing API consumers
to detect closed status without parsing the status enum or making
additional API calls.

app/views/cards/_card.json.jbuilder

@@ -6,6 +6,7 @@ json.cache! card do
 
   json.tags card.tags.pluck(:title).sort
 
+  json.closed card.closed?
   json.golden card.golden?
   json.last_active_at card.last_active_at.utc
   json.created_at card.created_at.utc

test/controllers/cards_controller_test.rb

@@ -145,6 +145,7 @@ class CardsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
 
     assert_equal card.title, @response.parsed_body["title"]
+    assert_equal card.closed?, @response.parsed_body["closed"]
     assert_equal 2, @response.parsed_body["steps"].size
   end