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/internal/handler/admin/ops_alerts_handler.go

@@ -430,4 +430,3 @@ func (h *OpsHandler) ListAlertEvents(c *gin.Context) {
 	}
 	response.Success(c, events)
 }
-

backend/internal/handler/admin/ops_settings_handler.go

@@ -146,4 +146,3 @@ func (h *OpsHandler) UpdateAdvancedSettings(c *gin.Context) {
 	}
 	response.Success(c, updated)
 }
-

backend/internal/handler/admin/ops_ws_handler.go

@@ -391,7 +391,7 @@ func tryAcquireOpsWSIPSlot(clientIP string, limit int32) bool {
 	}
 
 	v, _ := wsConnCountByIP.LoadOrStore(clientIP, &atomic.Int32{})
-	counter := v.(*atomic.Int32)
+	counter, ok := v.(*atomic.Int32); if !ok { return }
 
 	for {
 		current := counter.Load()
@@ -413,7 +413,7 @@ func releaseOpsWSIPSlot(clientIP string) {
 	if !ok {
 		return
 	}
-	counter := v.(*atomic.Int32)
+	counter, ok := v.(*atomic.Int32); if !ok { return }
 	next := counter.Add(-1)
 	if next <= 0 {
 		// Best-effort cleanup; safe even if a new slot was acquired concurrently.