fix: 修复 gosec 配置文件格式错误导致 CI 失败

gosec -conf 只支持 JSON 格式，将 .gosec.yaml 转换为 .gosec.json

.github/workflows/security-scan.yml

@@ -32,7 +32,7 @@ jobs:
         working-directory: backend
         run: |
           go install github.com/securego/gosec/v2/cmd/gosec@latest
-          gosec -conf .gosec.yaml -severity high -confidence high ./...
+          gosec -conf .gosec.json -severity high -confidence high ./...
 
   frontend-security:
     runs-on: ubuntu-latest