feat: add AI grading test endpoint and update connection logic

frontend/src/api/adminApi.ts

@@ -96,14 +96,16 @@ export function sendTestEmail(to: string | undefined, smtp: SmtpConfig): Promise
   })
 }
 
-export function testAiConnection(): Promise<GradingResult> {
-  return apiFetch('/api/grade', {
+export interface AiTestPayload {
+  provider: string
+  apiKey: string
+  model: string
+  baseURL?: string
+}
+
+export function testAiConnection(payload: AiTestPayload): Promise<GradingResult> {
+  return apiFetch('/api/admin/test-ai', {
     method: 'POST',
-    body: JSON.stringify({
-      type: 'short-answer',
-      question: { prompt: 'What is 2+2?', answer: '4' },
-      answer: '4',
-      language: 'en',
-    }),
+    body: JSON.stringify(payload),
   })
 }

frontend/src/components/AdminDashboard.tsx

@@ -135,7 +135,12 @@ export default function AdminDashboard({ onExit }: Readonly<Props>) {
     setTestAiSending(true);
     setTestAiMsg(null);
     try {
-      await adminApi.testAiConnection();
+      await adminApi.testAiConnection({
+        provider: settingsDraft.ai_provider,
+        apiKey: settingsDraft.ai_api_key,
+        model: settingsDraft.ai_model || 'gpt-4o-mini',
+        baseURL: settingsDraft.ai_base_url || undefined,
+      });
       setTestAiMsg({ ok: true, text: t("admin.aiGrading.testSuccess") });
     } catch (e) {
       setTestAiMsg({ ok: false, text: (e as Error).message });