Improve AI filtering error handling and visibility

- Add listAvailableModels() and validateModel() to openai.ts
- Improve testOpenAIConnection() to test configured model
- Add checkAIStatus endpoint to filtering router
- Add pre-execution AI config check in executeRules
- Improve error messages in AI filtering service (rate limit, quota, etc.)
- Add AI status warning banner on round detail page for filtering rounds

Now admins get clear errors when AI is misconfigured instead of silent flags.

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

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

@@ -84,6 +84,10 @@ function RoundDetailContent({ roundId }: { roundId: string }) {
     { roundId },
     { enabled: isFilteringRound }
   )
+  const { data: aiStatus } = trpc.filtering.checkAIStatus.useQuery(
+    { roundId },
+    { enabled: isFilteringRound }
+  )
 
   const utils = trpc.useUtils()
   const updateStatus = trpc.round.updateStatus.useMutation({
@@ -485,6 +489,22 @@ function RoundDetailContent({ roundId }: { roundId: string }) {
             </div>
           </CardHeader>
           <CardContent className="space-y-4">
+            {/* AI Status Warning */}
+            {aiStatus?.hasAIRules && !aiStatus?.configured && (
+              <div className="flex items-center gap-3 p-4 rounded-lg bg-amber-500/10 border border-amber-500/20">
+                <AlertTriangle className="h-5 w-5 text-amber-600 flex-shrink-0" />
+                <div className="flex-1">
+                  <p className="font-medium text-amber-700">AI Configuration Required</p>
+                  <p className="text-sm text-amber-600">
+                    {aiStatus.error || 'AI screening rules require OpenAI to be configured.'}
+                  </p>
+                </div>
+                <Button variant="outline" size="sm" asChild>
+                  <Link href="/admin/settings">Configure AI</Link>
+                </Button>
+              </div>
+            )}
+
             {/* Stats */}
             {filteringStats && filteringStats.total > 0 ? (
               <div className="grid gap-4 sm:grid-cols-4">