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.

lib/volumen/api_routes.rb

@@ -41,6 +41,10 @@ module Volumen
         feed_xml
       end
 
+      app.get "/api/volumen/feed.json" do
+        json(feed_json)
+      end
+
       app.get "/api/volumen/sitemap.xml" do
         content_type "application/xml"
         sitemap_xml

lib/volumen/server.rb

@@ -168,6 +168,41 @@ module Volumen
       end
     end
 
+    def feed_json
+      site = config.site
+      feed_url = "#{base_url}/api/volumen/feed.json"
+      {
+        "version" => "https://jsonfeed.org/version/1.1",
+        "title" => site["title"],
+        "home_page_url" => base_url,
+        "feed_url" => feed_url,
+        "description" => site["description"],
+        "language" => site["language"],
+        "items" => published_posts.first(20).map { |post| json_feed_item(post) }
+      }
+    end
+
+    def json_feed_item(post)
+      link = "#{base_url}/#{post.slug}"
+      item = {
+        "id" => link,
+        "url" => link,
+        "title" => post.title,
+        "content_html" => post.html,
+        "summary" => post.excerpt,
+        "date_published" => iso_date(post.metadata["date"]),
+        "tags" => post.tags
+      }
+      item.reject { |_k, v| v.nil? || v == "" || v == [] }
+    end
+
+    def iso_date(value)
+      case value
+      when Date then value.iso8601
+      when Time then value.to_date.iso8601
+      end
+    end
+
     def sitemap_xml
       urls = published_posts.map do |post|
         url = "  <url><loc>#{escape("#{base_url}/#{post.slug}")}</loc>"