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/post_test.rb

@@ -62,4 +62,19 @@ class PostTest < Minitest::Test
     post = Volumen::Post.new(metadata: { "slug" => "x" }, body: "short")
     assert_equal 1, post.reading_time
   end
+
+  def test_scheduled_post_is_not_scheduled_when_publish_at_is_past
+    post = Volumen::Post.new(metadata: { "publish_at" => Date.today - 1 })
+    refute_predicate post, :scheduled?
+  end
+
+  def test_scheduled_post_is_scheduled_when_publish_at_is_future
+    post = Volumen::Post.new(metadata: { "publish_at" => Date.today + 7 })
+    assert_predicate post, :scheduled?
+  end
+
+  def test_post_without_publish_at_is_not_scheduled
+    post = Volumen::Post.new(metadata: {})
+    refute_predicate post, :scheduled?
+  end
 end