Phase 1: warp_engine gem skeleton (path gem, mounted engine)

- libs/ruby/warp_engine: gemspec, WarpEngine::Engine (isolate_namespace with
  empty table_name_prefix), WarpEngine.configure surface (container paths,
  update_secret, image_owners), empty engine routes
- host Gemfile: path gem; host routes: mount WarpEngine::Engine => "/" as
  the last entry so host routes always win
- docker: api build context moved to repo root so libs/ is visible at
  bundle-install time; ./libs:/libs runtime mount; root .dockerignore to keep
  data/ and node_modules out of the build context

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/config/routes.rb

@@ -20,4 +20,7 @@ Rails.application.routes.draw do
   end
   get "update",       to: "update#update"
   get "file/*path",   to: "files#show", format: false
+
+  # Utolsó sor: a host route-jai nyernek, minden mást az engine kap el.
+  mount WarpEngine::Engine => "/"
 end