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>

.dockerignore

@@ -0,0 +1,7 @@
+# A repo-gyökér build contextből kihagyandó (az api image-nek csak apps/api + libs kell)
+data/
+apps/frontend/node_modules/
+apps/frontend/dist/
+apps/api/tmp/
+apps/api/log/
+.git/