Fix AI suggestions query running twice

Disable automatic refetching for expensive AI assignment query:
- Set staleTime to Infinity (never auto-refetch)
- Disable refetchOnWindowFocus, refetchOnReconnect, refetchOnMount
- Only manual refresh via button will trigger new request

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

src/app/(admin)/admin/rounds/[id]/assignments/page.tsx

@@ -89,10 +89,16 @@ function AssignmentManagementContent({ roundId }: { roundId: string }) {
     { enabled: !!round && !useAI }
   )
 
-  // AI-powered suggestions
+  // AI-powered suggestions (expensive - disable auto refetch)
   const { data: aiSuggestionsRaw, isLoading: loadingAI, refetch: refetchAI } = trpc.assignment.getAISuggestions.useQuery(
     { roundId, useAI: true },
-    { enabled: !!round && useAI }
+    {
+      enabled: !!round && useAI,
+      staleTime: Infinity, // Never consider stale (only refetch manually)
+      refetchOnWindowFocus: false,
+      refetchOnReconnect: false,
+      refetchOnMount: false,
+    }
   )
 
   // Normalize AI suggestions to match algorithmic format