功能: 修复并实现多节点集群部署 (#38)

基础修复：
- 新增节点离线检测：每 15s 扫描，超 45s 未心跳的远程节点自动置离线
- 节点删除前检查关联任务，避免孤立备份任务
- BackupTaskRepository 新增 CountByNodeID/ListByNodeID

Master 端 Agent 协议：
- 新增 AgentCommand 模型与命令队列仓储（pending/dispatched/succeeded/failed/timeout）
- 新增 AgentService：任务下发、命令轮询、结果回收、超时扫描
- 新增专用 Agent HTTP API（X-Agent-Token 认证）：
  /api/agent/heartbeat
  /api/agent/commands/poll
  /api/agent/commands/:id/result
  /api/agent/tasks/:id
  /api/agent/records/:id
- BackupExecutionService 支持 node 路由：task.NodeID 指向远程节点时自动入队派发

Agent CLI（backupx agent 子命令）：
- 配置：YAML 文件 / 环境变量 / CLI 参数，优先级 CLI > 文件 > 环境
- 心跳循环 + 命令轮询循环 + 优雅退出
- 本地复用 BackupRunner 与 storage registry 执行备份并直接上传
- 支持 run_task 和 list_dir 两种命令

远程目录浏览：
- NodeService 支持通过 Agent RPC 列出远程节点目录（15s 超时）

前端：
- NodesPage 添加节点后展示 Agent 启动命令和环境变量配置

文档：
- README 中英文重写"多节点集群"章节，含架构图、步骤、限制、CLI 参考

server/internal/repository/agent_command_repository_test.go

@@ -0,0 +1,120 @@
+package repository
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"backupx/server/internal/model"
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+	gormlogger "gorm.io/gorm/logger"
+)
+
+func newTestDB(t *testing.T) *gorm.DB {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{Logger: gormlogger.Default.LogMode(gormlogger.Silent)})
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	if err := db.AutoMigrate(&model.AgentCommand{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	return db
+}
+
+func TestAgentCommandRepository_ClaimPending(t *testing.T) {
+	db := newTestDB(t)
+	repo := NewAgentCommandRepository(db)
+	ctx := context.Background()
+
+	// 插入两条 pending 命令
+	c1 := &model.AgentCommand{NodeID: 5, Type: "run_task", Status: model.AgentCommandStatusPending, Payload: `{"taskId":1}`}
+	c2 := &model.AgentCommand{NodeID: 5, Type: "list_dir", Status: model.AgentCommandStatusPending, Payload: `{"path":"/"}`}
+	c3 := &model.AgentCommand{NodeID: 99, Type: "run_task", Status: model.AgentCommandStatusPending}
+	for _, c := range []*model.AgentCommand{c1, c2, c3} {
+		if err := repo.Create(ctx, c); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// 第一次 Claim 应拿到 c1
+	claimed, err := repo.ClaimPending(ctx, 5)
+	if err != nil {
+		t.Fatalf("claim: %v", err)
+	}
+	if claimed == nil || claimed.ID != c1.ID || claimed.Status != model.AgentCommandStatusDispatched {
+		t.Fatalf("expected c1 dispatched: %+v", claimed)
+	}
+
+	// 第二次应拿到 c2
+	claimed2, err := repo.ClaimPending(ctx, 5)
+	if err != nil || claimed2 == nil || claimed2.ID != c2.ID {
+		t.Fatalf("expected c2: %+v %v", claimed2, err)
+	}
+
+	// 第三次无 pending，返回 nil
+	claimed3, err := repo.ClaimPending(ctx, 5)
+	if err != nil || claimed3 != nil {
+		t.Fatalf("expected nil, got %+v", claimed3)
+	}
+
+	// 不同 node 的命令不应被抢到
+	other, err := repo.ClaimPending(ctx, 5)
+	if err != nil || other != nil {
+		t.Fatalf("expected nil: %+v", other)
+	}
+}
+
+func TestAgentCommandRepository_Update(t *testing.T) {
+	db := newTestDB(t)
+	repo := NewAgentCommandRepository(db)
+	ctx := context.Background()
+	cmd := &model.AgentCommand{NodeID: 1, Type: "run_task", Status: model.AgentCommandStatusPending}
+	_ = repo.Create(ctx, cmd)
+
+	cmd.Status = model.AgentCommandStatusSucceeded
+	cmd.Result = `{"ok":true}`
+	now := time.Now().UTC()
+	cmd.CompletedAt = &now
+	if err := repo.Update(ctx, cmd); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := repo.FindByID(ctx, cmd.ID)
+	if err != nil || got == nil {
+		t.Fatal(err)
+	}
+	if got.Status != model.AgentCommandStatusSucceeded || got.Result != `{"ok":true}` {
+		t.Errorf("mismatch: %+v", got)
+	}
+}
+
+func TestAgentCommandRepository_MarkStaleTimeout(t *testing.T) {
+	db := newTestDB(t)
+	repo := NewAgentCommandRepository(db)
+	ctx := context.Background()
+	old := time.Now().Add(-time.Hour)
+	recent := time.Now()
+	// 两条 dispatched：一条旧、一条新
+	oldCmd := &model.AgentCommand{NodeID: 1, Type: "run_task", Status: model.AgentCommandStatusDispatched, DispatchedAt: &old}
+	newCmd := &model.AgentCommand{NodeID: 1, Type: "run_task", Status: model.AgentCommandStatusDispatched, DispatchedAt: &recent}
+	_ = repo.Create(ctx, oldCmd)
+	_ = repo.Create(ctx, newCmd)
+
+	n, err := repo.MarkStaleTimeout(ctx, time.Now().Add(-30*time.Minute))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 1 {
+		t.Errorf("expected 1 row, got %d", n)
+	}
+	oldGot, _ := repo.FindByID(ctx, oldCmd.ID)
+	newGot, _ := repo.FindByID(ctx, newCmd.ID)
+	if oldGot.Status != model.AgentCommandStatusTimeout {
+		t.Errorf("old should be timeout: %+v", oldGot)
+	}
+	if newGot.Status != model.AgentCommandStatusDispatched {
+		t.Errorf("new should stay dispatched: %+v", newGot)
+	}
+}