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_test.go

@@ -88,6 +88,71 @@ func TestKubernetesClientReadsSiteAndMutatesOnlyWorkerScale(t *testing.T) {
 	}
 }
 
+func TestKubernetesClientAllowsDisabledSiteWithoutWorkerNodes(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, request *http.Request) {
+		switch {
+		case request.Method == http.MethodGet && strings.Contains(request.URL.Path, "/deployments/easyai-worker-ningbo"):
+			_, _ = w.Write([]byte(`{
+				"spec":{"replicas":0,"template":{"spec":{"containers":[{
+					"name":"worker","image":"registry.invalid/gateway@sha256:abc",
+					"env":[{"name":"AI_GATEWAY_REVISION","value":"release-sha"}],
+					"resources":{"requests":{"memory":"1536Mi","cpu":"500m"}}
+				}]}}}
+			}`))
+		case request.Method == http.MethodGet && request.URL.Path == "/api/v1/nodes":
+			_, _ = w.Write([]byte(`{"items":[]}`))
+		default:
+			http.NotFound(w, request)
+		}
+	}))
+	defer server.Close()
+	tokenFile := filepath.Join(t.TempDir(), "token")
+	if err := os.WriteFile(tokenFile, []byte("test-token"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	client, err := NewKubernetesClient(KubernetesConfig{
+		Namespace: "easyai", APIServer: server.URL, TokenFile: tokenFile, HTTPClient: server.Client(),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	state, err := client.SiteState(context.Background(), "ningbo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if state.Site != "ningbo" || state.CurrentReplicas != 0 || len(state.Nodes) != 0 {
+		t.Fatalf("site state=%+v", state)
+	}
+}
+
+func TestKubernetesClientRejectsActiveSiteWithoutWorkerNodes(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, request *http.Request) {
+		switch {
+		case request.Method == http.MethodGet && strings.Contains(request.URL.Path, "/deployments/easyai-worker-ningbo"):
+			_, _ = w.Write([]byte(`{"spec":{"replicas":1,"template":{"spec":{"containers":[]}}}}`))
+		case request.Method == http.MethodGet && request.URL.Path == "/api/v1/nodes":
+			_, _ = w.Write([]byte(`{"items":[]}`))
+		default:
+			http.NotFound(w, request)
+		}
+	}))
+	defer server.Close()
+	tokenFile := filepath.Join(t.TempDir(), "token")
+	if err := os.WriteFile(tokenFile, []byte("test-token"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	client, err := NewKubernetesClient(KubernetesConfig{
+		Namespace: "easyai", APIServer: server.URL, TokenFile: tokenFile, HTTPClient: server.Client(),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := client.SiteState(context.Background(), "ningbo"); err == nil ||
+		!strings.Contains(err.Error(), "site ningbo has no matching nodes") {
+		t.Fatalf("expected missing Worker node error, got %v", err)
+	}
+}
+
 func TestQuantityParsing(t *testing.T) {
 	if value, err := parseBinaryQuantity("1536Mi"); err != nil || value != 1536<<20 {
 		t.Fatalf("memory=%d err=%v", value, err)