Add a runnable example compose stack for WarpEngine

examples/compose boots everything the engine's workflow assumes: mysql, a
minimal Rails host consuming the engine as a path gem, an SSH drop area
sharing the softwares volume with the app, and — behind the ci profile —
gitea plus woodpecker (agent attached to the stack network so pipeline
steps reach droparea/app by service name).

host_app doubles as a reference for a brand-new host: Gemfile, the two
initializers, apipie + engine mounts in routes.rb; on first boot the
entrypoint runs the install generator and db:prepare.

The README gains a detailed bring-up walkthrough: quickstart, publishing
a release by hand over scp + /update (verified end to end from a clean
slate), and the full gitea/woodpecker OAuth wiring.

examples/compose/host_app/config/initializers/warp_engine.rb

@@ -0,0 +1,9 @@
+Rails.application.config.to_prepare do
+  WarpEngine.configure do |c|
+    c.file_container_path  = ENV.fetch("FILE_CONTAINER_PATH", "/softwares")
+    c.image_container_path = ENV.fetch("IMAGE_CONTAINER_PATH", "/images")
+
+    # Beállítatlan secret esetén a /update endpoint minden kérést elutasít.
+    c.update_secret = ENV["UPDATE_SECRET"]
+  end
+end