feat(ws): A3 WebSocket 统一连接管理 — 心跳/注册表/监控/优雅关闭

- 新增 pkg/wshub/manager.go 统一 WebSocket 连接管理器 (WSManager)
- 6 个 WebSocket handler 全部迁移: WebShell/kubectl/PodExec/Audit/Metrics/Pipeline
- 心跳机制: ping(30s) + pong handler + read deadline(60s) 自动检测死连接
- 连接注册表: 元数据(类型/用户/远程地址/连接时间/最后活跃)
- 每用户连接限制 MaxConnsPerUser=15 防资源耗尽
- 监控端点: GET /api/v1/ws/stats + GET /api/v1/ws/connections
- 优雅关闭: CloseAll() 发送 CloseGoingAway 帧并关闭所有连接
- 消除 3 个重复 websocket.Upgrader 变量统一为 WSManager 内部 upgrader
- 6/6 A3 测试通过, 17/17 回归测试通过

backend/controller/k8s_pod.go

@@ -11,8 +11,10 @@ import (
 	"strings"
 	"time"
 
+	"ezops/middleware"
 	"ezops/model"
 	k8sclient "ezops/pkg/kubernetesclient"
+	"ezops/pkg/wshub"
 
 	"github.com/gin-gonic/gin"
 	"github.com/gorilla/websocket"
@@ -212,12 +214,15 @@ func (h *KubernetesHandler) ExecTerminal(c *gin.Context) {
 	}
 	defer k8sConn.Close()
 
-	// Upgrade frontend connection to WebSocket
-	frontendConn, err := upgrader.Upgrade(c.Writer, c.Request, nil)
+	// Upgrade frontend connection to WebSocket (tracked by WSManager)
+	userID := middleware.GetUserID(c)
+	username := middleware.GetUsername(c)
+	frontendConn, info, err := h.wsMgr.Upgrade(c, wshub.TypePodExec, userID, username)
 	if err != nil {
 		return
 	}
-	defer frontendConn.Close()
+	defer h.wsMgr.Unregister(info)
+	h.wsMgr.StartHeartbeat(frontendConn, info)
 
 	done := make(chan struct{}, 2)