feat: add JSON Feed endpoint at /api/volumen/feed.json

Implements jsonfeed.org version 1.1 with title, home_page_url, feed_url,
description, language, and items with content_html and date_published.

test/server_test.rb

@@ -141,4 +141,16 @@ class ServerTest < Minitest::Test
     slugs = JSON.parse(last_response.body)["posts"].map { |post| post["slug"] }
     refute_includes slugs, "later"
   end
+
+  def test_json_feed_endpoint
+    get "/api/volumen/feed.json"
+    assert_predicate last_response, :ok?
+    data = JSON.parse(last_response.body)
+    assert_equal "https://jsonfeed.org/version/1.1", data["version"]
+    assert_equal "My Blog", data["title"]
+    assert_equal 1, data["items"].length
+    assert_equal "Hello", data["items"].first["title"]
+    assert_includes data["items"].first["content_html"], "<strong>world</strong>"
+    refute_includes data["items"].map { |i| i["title"] }, "Draft"
+  end
 end