Fix metrics API response structure

Backend returns metrics as an array, not an object.
- Update MetricsResponse type to use MetricDefinition[] instead of Record
- Remove Object.values() conversion in compare and rankings pages
- Fix useMetrics hook to handle array instead of object
- Fix getMetric to use array.find() instead of object indexing

Fixes empty metric dropdown on compare page.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextjs-app/app/rankings/page.tsx

@@ -43,8 +43,8 @@ export default async function RankingsPage({ searchParams }: RankingsPageProps)
       fetchMetrics(),
     ]);
 
-    // Convert metrics object to array
-    const metricsArray = Object.values(metricsResponse?.metrics || {});
+    // Metrics is already an array
+    const metricsArray = metricsResponse?.metrics || [];
 
     return (
       <RankingsView