fix: 修复批量更新凭证明细与缓存TTL抖动

- BatchUpdateCredentials 返回 success/failed/results 及 success_ids/failed_ids

- billing jitteredTTL 改为只减不增，确保TTL不超上界

- crypto/rand 失败时随机ID降级避免 panic

- OpenAI SelectAccount 失败日志去重并补充字段

- 修复两处类型断言以通过 errcheck

backend/internal/service/sse_scanner_buffer_pool.go

@@ -13,7 +13,12 @@ var sseScannerBuf64KPool = sync.Pool{
 }
 
 func getSSEScannerBuf64K() *sseScannerBuf64K {
-	return sseScannerBuf64KPool.Get().(*sseScannerBuf64K)
+	v := sseScannerBuf64KPool.Get()
+	buf, ok := v.(*sseScannerBuf64K)
+	if !ok || buf == nil {
+		return new(sseScannerBuf64K)
+	}
+	return buf
 }
 
 func putSSEScannerBuf64K(buf *sseScannerBuf64K) {