fix: Critical 보안 이슈 4건 수정

1. SQL injection 취약점 수정 (currency 직접 삽입 제거)
   - SQL 쿼리에서 currency 제거, 결과 매핑에서 추가

2. 에러 메시지 정보 노출 수정
   - 클라이언트에 내부 에러 상세 숨김
   - 서버 로그에만 기록

3. API 키 로깅 제거
   - sk-*** 형식만 표시, 실제 값 노출 안함

4. Rate limit fail-closed 정책 적용
   - KV 오류 시 요청 거부 (보안 강화)

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

src/index.ts

@@ -635,8 +635,8 @@ async function checkRateLimit(clientIP: string, env: Env): Promise<{ allowed: bo
     return { allowed: true, requestId };
   } catch (error) {
     console.error('[RateLimit] KV error:', error);
-    // On error, allow the request (fail open)
-    return { allowed: true, requestId };
+    // On error, deny the request (fail closed) for security
+    return { allowed: false, requestId };
   }
 }
 
@@ -1077,10 +1077,10 @@ async function handleRecommend(
   } catch (error) {
     console.error('[Recommend] Error:', error);
     console.error('[Recommend] Error stack:', error instanceof Error ? error.stack : 'No stack');
+    console.error('[Recommend] Error details:', error instanceof Error ? error.message : 'Unknown error');
     return jsonResponse(
       {
         error: 'Failed to generate recommendations',
-        details: error instanceof Error ? error.message : 'Unknown error',
         request_id: requestId,
       },
       500,
@@ -1303,7 +1303,6 @@ async function queryCandidateServers(
       it.gpu_count,
       it.gpu_type,
       MIN(${priceColumn}) as monthly_price,
-      '${currency}' as currency,
       r.region_name as region_name,
       r.region_code as region_code,
       r.country_code as country_code
@@ -1436,8 +1435,14 @@ async function queryCandidateServers(
     throw new Error('Failed to query candidate servers');
   }
 
-  // Validate each result with type guard
-  const validServers = (result.results as unknown[]).filter(isValidServer);
+  // Add currency to each result and validate with type guard
+  const serversWithCurrency = (result.results as unknown[]).map(server => {
+    if (typeof server === 'object' && server !== null) {
+      return { ...server, currency };
+    }
+    return server;
+  });
+  const validServers = serversWithCurrency.filter(isValidServer);
   const invalidCount = result.results.length - validServers.length;
   if (invalidCount > 0) {
     console.warn(`[Candidates] Filtered out ${invalidCount} invalid server records`);
@@ -1851,7 +1856,7 @@ async function getAIRecommendations(
     console.error('[AI] OPENAI_API_KEY has invalid format (should start with sk-)');
     throw new Error('Invalid OPENAI_API_KEY format');
   }
-  console.log('[AI] API key validated (sk-***' + apiKey.slice(-4) + ')');
+  console.log('[AI] API key validated (format: sk-***)');
 
   // Build dynamic tech specs prompt from database
   const techSpecsPrompt = formatTechSpecsForPrompt(techSpecs);