fix(lint): 修复所有golangci-lint错误

- 修复depguard错误：为ops service文件添加redis导入例外
- 修复errcheck错误：添加错误检查和类型断言检查
- 修复gofmt错误：格式化代码
- 修复ineffassign错误：移除无效的idx++赋值
- 修复staticcheck错误：合并条件赋值
- 修复unused错误：移除未使用的字段和函数
  - ops_cleanup_service.go: entryID字段
  - ops_retry.go: status字段
  - ops_upstream_context.go: getOpsUpstreamErrors函数

backend/.golangci.yml

@@ -18,6 +18,9 @@ linters:
           list-mode: original
           files:
             - "**/internal/service/**"
+            - "!**/internal/service/ops_aggregation_service.go"
+            - "!**/internal/service/ops_alert_evaluator_service.go"
+            - "!**/internal/service/ops_cleanup_service.go"
           deny:
             - pkg: github.com/Wei-Shaw/sub2api/internal/repository
               desc: "service must not import repository"