Serve engine-tagged wiki pages on a new /engines section

Frontend: /engines index + /engines/:slug detail routes, nav menu item and
en/hu translations. Engine pages are few, so the index uses an emphasized
poster-style design (dark slate, emerald accents, numbered full-width cards
with content preview) instead of the blog/howtos layouts. Slugs are the last
wiki path segment, since engine pages live scattered in the wiki tree.

API: /api/rss/engines feed linking to the site's engine pages, and a
WikiService#pages alias for #index — RssService called the alias-less name,
so the blog and howtos feeds were raising NoMethodError.

apps/api/config/routes.rb

@@ -11,6 +11,7 @@ Rails.application.routes.draw do
     get "rss/blog",             to: "rss#blog"
     get "rss/releases",         to: "rss#releases"
     get "rss/howtos",           to: "rss#howtos"
+    get "rss/engines",          to: "rss#engines"
   end
 
   # Utolsó sor: a host route-jai nyernek, a katalógus-útvonalakat