My earlier patch inserted the new handler between the existing
/broker/model-leaderboard decorator and its function, leaving two routes
pointing at model-leaderboard and none at /broker/metrics, which 404'd. The
dashboard's co-op leaderboard was being served the wrong payload.
Caught by the post-deploy check (the 404 on a route whose code was present in
the container), not by the build. Adds a pre-build route-table assertion —
every path must be followed by its intended handler — so this shape of mistake
fails before a build rather than after a deploy.