refactor: 消除集群执行服务冗余逻辑 + 修复节点状态滞后缺陷 (#74)

抽取备份/恢复/验证/复制四服务的复制粘贴逻辑到 execution_helpers.go（净减约 250 行）；节点状态改为按 LastSeen 实时推导，消除过期 online 误判；Agent systemd 单元补齐 LimitNOFILE 与单机端一致。go build/test 全绿。

server/internal/model/node_status_test.go

@@ -0,0 +1,58 @@
+package model
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNodeEffectiveStatus(t *testing.T) {
+	now := time.Date(2026, 5, 26, 12, 0, 0, 0, time.UTC)
+	cases := []struct {
+		name string
+		node *Node
+		want string
+	}{
+		{
+			name: "remote fresh heartbeat → stored online",
+			node: &Node{IsLocal: false, Status: NodeStatusOnline, LastSeen: now.Add(-10 * time.Second)},
+			want: NodeStatusOnline,
+		},
+		{
+			name: "remote stale heartbeat but stored online → derived offline",
+			node: &Node{IsLocal: false, Status: NodeStatusOnline, LastSeen: now.Add(-90 * time.Second)},
+			want: NodeStatusOffline,
+		},
+		{
+			name: "remote just past grace period → offline",
+			node: &Node{IsLocal: false, Status: NodeStatusOnline, LastSeen: now.Add(-(OfflineGracePeriod + time.Second))},
+			want: NodeStatusOffline,
+		},
+		{
+			name: "remote within grace period → online",
+			node: &Node{IsLocal: false, Status: NodeStatusOnline, LastSeen: now.Add(-(OfflineGracePeriod - time.Second))},
+			want: NodeStatusOnline,
+		},
+		{
+			name: "local node ignores LastSeen → stored online",
+			node: &Node{IsLocal: true, Status: NodeStatusOnline, LastSeen: now.Add(-24 * time.Hour)},
+			want: NodeStatusOnline,
+		},
+		{
+			name: "remote stored offline stays offline",
+			node: &Node{IsLocal: false, Status: NodeStatusOffline, LastSeen: now.Add(-5 * time.Second)},
+			want: NodeStatusOffline,
+		},
+		{
+			name: "nil node → offline",
+			node: nil,
+			want: NodeStatusOffline,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := tc.node.EffectiveStatus(now); got != tc.want {
+				t.Fatalf("EffectiveStatus = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}