perf: memoize published_posts within a single request

Store the filtered and sorted list of published posts in an instance
variable so repeated calls during the same request (posts list, tags,
feeds, sitemap) reuse the cached result instead of re-filtering and
re-sorting from scratch each time. Sinatra resets instance variables
between requests, so the cache stays fresh.

lib/volumen/server.rb

@@ -79,8 +79,8 @@ module Volumen
     end
 
     def published_posts
-      store.all.reject { |post| post.draft? || post.scheduled? }
+      @published_posts ||= store.all.reject { |post| post.draft? || post.scheduled? }
-               .sort_by { |post| post.date_string || "" }.reverse
+                                  .sort_by { |post| post.date_string || "" }.reverse
     end
 
     def site_payload