perf: add Cache-Control header to public API responses

Set Cache-Control: public, max-age=60 on all /api/volumen/* endpoints
so browsers and CDNs can cache responses for 60 seconds, reducing
unnecessary re-fetches.

lib/volumen/api_routes.rb

@@ -5,7 +5,8 @@ module Volumen
   module ApiRoutes
     def self.registered(app)
       app.before "/api/volumen/*" do
-        headers "Access-Control-Allow-Origin" => "*"
+        headers "Access-Control-Allow-Origin" => "*",
+                "Cache-Control" => "public, max-age=60"
         content_type :json
       end