feat(gateway): add Kong route for app version check endpoint

Add /api/app/version route to Kong declarative config so that the
Flutter app's GET /api/app/version/check?platform=&current_version_code=
request can reach version-service through the API gateway.

Previously only /api/v1/versions was routed; the public check endpoint
served by AppVersionCheckController was unreachable (Kong returned
"no Route matched with those values").

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

packages/gateway/config/kong.yml

@@ -120,6 +120,10 @@ services:
         paths:
           - /api/v1/versions
         strip_path: false
+      - name: app-version-check-route
+        paths:
+          - /api/app/version
+        strip_path: false
 
   - name: billing-service
     url: http://billing-service:3010