portal: fix the broker metrics route (decorator landed on the wrong handler)

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.

app/main.py

@@ -3480,7 +3480,7 @@ async def broker_usage(request: Request, days: int = 0):
     return await broker_get_usage(email, days=days)
 
 
-@app.get("/broker/model-leaderboard")
+@app.get("/broker/metrics")
 @limiter.limit("30/minute")
 async def broker_metrics(request: Request, days: int = 90):
     """Co-op metrics over time, for the member dashboard.