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/hooks/useMetrics.ts

@@ -19,9 +19,9 @@ export function useMetrics() {
   );
 
   return {
-    metrics: data?.metrics || {},
-    metricsList: data?.metrics ? Object.values(data.metrics) : [],
-    getMetric: (key: string) => data?.metrics?.[key],
+    metrics: data?.metrics || [],
+    metricsList: data?.metrics || [],
+    getMetric: (key: string) => data?.metrics?.find(m => m.key === key),
     isLoading,
     error,
   };