feat: add scheduled publishing with publish_at frontmatter field

Posts with a future publish_at date are hidden from the public API
and feeds, same as drafts. The admin form exposes a Publish at field.

test/server_test.rb

@@ -126,4 +126,19 @@ class ServerTest < Minitest::Test
     assert_equal "*", last_response.headers["Access-Control-Allow-Origin"]
     assert_equal "GET, OPTIONS", last_response.headers["Access-Control-Allow-Methods"]
   end
+
+  def test_scheduled_post_is_hidden
+    future = (Date.today + 30).strftime("%Y-%m-%d")
+    File.write(File.join(@dir, "later.md"), <<~POST)
+      +++
+      title = "Later"
+      publish_at = #{future}
+      +++
+
+      Not yet.
+    POST
+    get "/api/volumen/posts"
+    slugs = JSON.parse(last_response.body)["posts"].map { |post| post["slug"] }
+    refute_includes slugs, "later"
+  end
 end