Wire federation service stub and update shared submodule

- Register StubFederationService in services/__init__.py
- Add federation to CI sibling list
- Add federation URL to app-config.yaml
- Update shared submodule with federation models/contracts/services

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/app-config.yaml

@@ -13,6 +13,7 @@ app_urls:
   market: "http://localhost:8001"
   cart: "http://localhost:8002"
   events: "http://localhost:8003"
+  federation: "http://localhost:8004"
 cache:
   fs_root: _snapshot  # <- absolute path to your snapshot dir
 categories: