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/Gemfile.lock

@@ -1,3 +1,9 @@
+PATH
+  remote: ../libs/ruby/warp_engine
+  specs:
+    warp_engine (0.1.0)
+      rails (>= 8.0)
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -380,6 +386,7 @@ DEPENDENCIES
   sassc-rails
   shoulda-matchers (~> 6.0)
   sprockets-rails
+  warp_engine!
 
 BUNDLED WITH
    2.5.22