fix(capacity): 允许显式禁用无节点站点

当 Worker Deployment 为零副本且站点没有 eligible Worker 节点时返回零容量状态，避免 min/max=0 的禁用站点令容量控制器持续 NotReady；非零副本仍保持失败关闭。\n\n验证：容量控制器单测、Go 全量测试、go vet、gofmt、迁移安全检查和人工发布脚本测试。

apps/api/internal/capacitycontroller/kubernetes.go

@@ -179,6 +179,13 @@ func (client *KubernetesClient) SiteState(ctx context.Context, site string) (Kub
 		return KubernetesSiteState{}, err
 	}
 	if len(nodes.Items) == 0 {
+		// A zero-replica Deployment with no eligible nodes represents an
+		// explicitly disabled site. It must remain visible to the planner, but it
+		// has no node or metrics budget to collect. A non-zero Deployment still
+		// fails closed so replicas cannot disappear from capacity accounting.
+		if state.CurrentReplicas == 0 {
+			return state, nil
+		}
 		return KubernetesSiteState{}, fmt.Errorf("site %s has no matching nodes", site)
 	}
 	var pods struct {