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>

warp_engine.gemspec

@@ -0,0 +1,21 @@
+require_relative "lib/warp_engine/version"
+
+Gem::Specification.new do |spec|
+  spec.name        = "warp_engine"
+  spec.version     = WarpEngine::VERSION
+  spec.authors     = [ "Teletype Games" ]
+  spec.summary     = "Retro software catalog engine with a CI-callable release updater"
+  spec.description = "Mountable Rails engine providing a software catalog (softwares, releases, " \
+                     "release assets, images, platform links), a pipeline-callable update endpoint, " \
+                     "a public read-only JSON API and ActiveAdmin resources for a host-owned admin."
+  spec.license     = "MIT"
+  spec.homepage    = "https://teletypegames.org"
+
+  spec.required_ruby_version = ">= 3.2"
+
+  spec.metadata["allowed_push_host"] = "https://git.teletypegames.org"
+
+  spec.files = Dir["{app,config,db,lib}/**/*", "README.md"]
+
+  spec.add_dependency "rails", ">= 8.0"
+end