feat(security): 敏感操作 step-up 2FA 开关化，安全开关默认关闭

新增系统设置 step_up_enabled（默认关闭），把敏感操作 2FA 门控做成可开关；
同时将会话 IP/UA 绑定默认值从开启改为关闭，避免用户因 IP 变动登录后掉线。

## 新增功能
- 敏感操作 step-up 2FA 总开关 step_up_enabled（默认关闭）：关闭时账号/代理导出、
  备份创建/下载、S3 配置修改、提升管理员等操作恢复门控引入前的直接放行行为；
  开启后要求当前会话在 15 分钟内完成过 TOTP step-up 验证。

## 优化改进
- 会话 IP/UA 绑定默认改为关闭（功能保留，可在设置页按需开启）。
- 开启 step-up 开关需操作者本人已启用 TOTP（防自锁）；关闭开关本身作为敏感操作，
  需通过 step-up 验证（防止攻击者拿到会话后先关闸再导出/备份）。
- 两个安全开关请求字段改为可空指针（省略=保持现值），避免旧客户端全量保存时
  静默重置安全开关。
- 备份恢复（整库覆盖可回滚安全设置）纳入 step-up 门控。
- 审计摘要 diffSettings 补记 step_up_enabled / session_binding_enabled 变更。

## Bug 修复
- 修复 BackupView 恢复操作 409（恢复进行中）判断未适配 apiClient 扁平化错误对象。

backend/internal/handler/admin/admin_basic_handlers_test.go

@@ -16,7 +16,7 @@ func setupAdminRouter() (*gin.Engine, *stubAdminService) {
 	router := gin.New()
 	adminSvc := newStubAdminService()
 
-	userHandler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil)
+	userHandler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil, nil)
 	groupHandler := NewGroupHandler(adminSvc, nil, nil)
 	proxyHandler := NewProxyHandler(adminSvc)
 	redeemHandler := NewRedeemHandler(adminSvc, nil)

backend/internal/handler/admin/setting_handler.go

@@ -59,6 +59,8 @@ type SettingHandler struct {
 	paymentService           *service.PaymentService
 	userAttributeService     *service.UserAttributeService
 	notificationEmailService *service.NotificationEmailService
+	totpService              *service.TotpService
+	userService              *service.UserService
 }
 
 // NewSettingHandler 创建系统设置处理器
@@ -80,6 +82,15 @@ func (h *SettingHandler) SetNotificationEmailService(notificationEmailService *s
 	h.notificationEmailService = notificationEmailService
 }
 
+// SetStepUpDeps attaches the services backing the step-up switch preconditions
+// (enable requires the acting admin to have TOTP enabled; disable is itself a
+// step-up gated operation), without changing the constructor signature used by
+// existing unit tests.
+func (h *SettingHandler) SetStepUpDeps(totpService *service.TotpService, userService *service.UserService) {
+	h.totpService = totpService
+	h.userService = userService
+}
+
 // GetSettings 获取所有系统设置
 // GET /api/v1/admin/settings
 func (h *SettingHandler) GetSettings(c *gin.Context) {
@@ -124,6 +135,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		TotpEnabled:                                            settings.TotpEnabled,
 		TotpEncryptionKeyConfigured:                            h.settingService.IsTotpEncryptionKeyConfigured(),
 		SessionBindingEnabled:                                  settings.SessionBindingEnabled,
+		StepUpEnabled:                                          settings.StepUpEnabled,
 		AuditLogRetentionDays:                                  settings.AuditLogRetentionDays,
 		LoginAgreementEnabled:                                  settings.LoginAgreementEnabled,
 		LoginAgreementMode:                                     settings.LoginAgreementMode,

backend/internal/handler/admin/setting_handler_audit.go

@@ -56,6 +56,12 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.TotpEnabled != after.TotpEnabled {
 		changed = append(changed, "totp_enabled")
 	}
+	if before.SessionBindingEnabled != after.SessionBindingEnabled {
+		changed = append(changed, "session_binding_enabled")
+	}
+	if before.StepUpEnabled != after.StepUpEnabled {
+		changed = append(changed, "step_up_enabled")
+	}
 	if before.LoginAgreementEnabled != after.LoginAgreementEnabled {
 		changed = append(changed, "login_agreement_enabled")
 	}

backend/internal/handler/admin/setting_handler_stepup_switch_test.go

@@ -0,0 +1,157 @@
+package admin
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+// step-up 开关转换的门控测试。
+// 测试环境不注入认证上下文/userService，因此一旦触发校验会以 401/403/500 中止；
+// 借此区分「触发了转换校验」与「直接放行到常规保存（200）」。
+
+func newStepUpSwitchTestHandler(t *testing.T, stored map[string]string) (*SettingHandler, *settingHandlerRepoStub) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	repo := &settingHandlerRepoStub{values: stored}
+	svc := service.NewSettingService(repo, &config.Config{Default: config.DefaultConfig{UserConcurrency: 5}})
+	return NewSettingHandler(svc, nil, nil, nil, nil, nil, nil), repo
+}
+
+func doUpdateSettings(t *testing.T, h *SettingHandler, body map[string]any, prepare func(c *gin.Context)) *httptest.ResponseRecorder {
+	t.Helper()
+	rawBody, err := json.Marshal(body)
+	require.NoError(t, err)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPut, "/api/v1/admin/settings", bytes.NewReader(rawBody))
+	c.Request.Header.Set("Content-Type", "application/json")
+	if prepare != nil {
+		prepare(c)
+	}
+
+	h.UpdateSettings(c)
+	return rec
+}
+
+// 开启开关（false→true）：无认证上下文时拒绝，且带专用错误标记。
+func TestUpdateSettingsEnableStepUpRejectsWithoutSession(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": true}, nil)
+
+	require.Equal(t, http.StatusForbidden, rec.Code)
+	require.Contains(t, rec.Body.String(), "STEP_UP_ENABLE_REQUIRES_TOTP")
+	require.NotEqual(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+}
+
+// 开启开关：admin API key（机器凭证）一律拒绝，reason 与门控保持一致便于前端分流。
+func TestUpdateSettingsEnableStepUpRejectsAdminAPIKey(t *testing.T) {
+	h, _ := newStepUpSwitchTestHandler(t, map[string]string{})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": true}, func(c *gin.Context) {
+		c.Set("auth_method", service.AuditAuthMethodAdminAPIKey)
+	})
+
+	require.Equal(t, http.StatusForbidden, rec.Code)
+	require.Contains(t, rec.Body.String(), "STEP_UP_ADMIN_API_KEY_FORBIDDEN")
+}
+
+// 开启开关：有认证会话但 userService 未注入时 fail-closed（500），不得放行。
+func TestUpdateSettingsEnableStepUpFailsClosedWithoutUserService(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": true}, func(c *gin.Context) {
+		c.Set(string(middleware.ContextKeyUser), middleware.AuthSubject{UserID: 1})
+	})
+
+	require.Equal(t, http.StatusInternalServerError, rec.Code)
+	require.NotEqual(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+}
+
+// 关闭开关（true→false）本身是敏感操作：无认证上下文时被 step-up 门控以 401 拦截。
+func TestUpdateSettingsDisableStepUpRequiresStepUp(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{
+		service.SettingKeyStepUpEnabled: "true",
+	})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": false}, nil)
+
+	require.Equal(t, http.StatusUnauthorized, rec.Code)
+	require.Equal(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+}
+
+// 关闭开关：admin API key 被 step-up 门控以 403 拦截。
+func TestUpdateSettingsDisableStepUpRejectsAdminAPIKey(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{
+		service.SettingKeyStepUpEnabled: "true",
+	})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": false}, func(c *gin.Context) {
+		c.Set("auth_method", service.AuditAuthMethodAdminAPIKey)
+	})
+
+	require.Equal(t, http.StatusForbidden, rec.Code)
+	require.Contains(t, rec.Body.String(), "STEP_UP_ADMIN_API_KEY_FORBIDDEN")
+	require.Equal(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+}
+
+// 无状态转换（false→false）：不触发任何转换校验，常规保存成功且默认持久化为 false。
+func TestUpdateSettingsStepUpNoTransitionSkipsGate(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": false}, nil)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "false", repo.values[service.SettingKeyStepUpEnabled])
+	// 会话 IP/UA 绑定默认关闭：未显式提交时持久化 false。
+	require.Equal(t, "false", repo.values[service.SettingKeySessionBindingEnabled])
+}
+
+// 保持开启（true→true）：不触发转换校验，常规保存不被打断。
+func TestUpdateSettingsStepUpKeepEnabledSkipsGate(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{
+		service.SettingKeyStepUpEnabled: "true",
+	})
+
+	rec := doUpdateSettings(t, h, map[string]any{"step_up_enabled": true}, nil)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+}
+
+// 省略字段=保持现值：不含 step_up_enabled/session_binding_enabled 的旧客户端全量保存
+// 不得把已开启的安全开关静默重置，也不触发任何转换门控。
+func TestUpdateSettingsOmittedSecuritySwitchesKeepStoredValues(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{
+		service.SettingKeyStepUpEnabled:         "true",
+		service.SettingKeySessionBindingEnabled: "true",
+	})
+
+	rec := doUpdateSettings(t, h, map[string]any{"registration_enabled": true}, nil)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "true", repo.values[service.SettingKeyStepUpEnabled])
+	require.Equal(t, "true", repo.values[service.SettingKeySessionBindingEnabled])
+}
+
+// 省略字段在开关本就关闭时同样保持关闭（默认值路径）。
+func TestUpdateSettingsOmittedSecuritySwitchesKeepDisabled(t *testing.T) {
+	h, repo := newStepUpSwitchTestHandler(t, map[string]string{})
+
+	rec := doUpdateSettings(t, h, map[string]any{"registration_enabled": true}, nil)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "false", repo.values[service.SettingKeyStepUpEnabled])
+	require.Equal(t, "false", repo.values[service.SettingKeySessionBindingEnabled])
+}

backend/internal/handler/admin/setting_handler_update.go

@@ -11,6 +11,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -27,7 +28,8 @@ type UpdateSettingsRequest struct {
 	FrontendURL                      string                       `json:"frontend_url"`
 	InvitationCodeEnabled            bool                         `json:"invitation_code_enabled"`
 	TotpEnabled                      bool                         `json:"totp_enabled"`             // TOTP 双因素认证
-	SessionBindingEnabled            bool                         `json:"session_binding_enabled"`  // 会话 IP/UA 绑定
+	SessionBindingEnabled            *bool                        `json:"session_binding_enabled"`  // 会话 IP/UA 绑定（省略=保持现值）
+	StepUpEnabled                    *bool                        `json:"step_up_enabled"`          // 敏感操作 step-up 2FA（省略=保持现值）
 	AuditLogRetentionDays            int                          `json:"audit_log_retention_days"` // 审计日志保留天数
 	LoginAgreementEnabled            bool                         `json:"login_agreement_enabled"`
 	LoginAgreementMode               string                       `json:"login_agreement_mode"`
@@ -335,6 +337,41 @@ type UpdateSettingsRequest struct {
 
 // UpdateSettings 更新系统设置
 // PUT /api/v1/admin/settings
+// ensureActorTotpForStepUp 校验当前操作者具备开启 step-up 门控的条件：
+// 必须是真人管理员会话（admin API key 无法完成 TOTP step-up，拒绝）且本人已启用 TOTP。
+// 校验失败时写入错误响应并返回 false。
+func (h *SettingHandler) ensureActorTotpForStepUp(c *gin.Context) bool {
+	if c.GetString("auth_method") == service.AuditAuthMethodAdminAPIKey {
+		response.ErrorWithDetails(c, http.StatusForbidden,
+			"Admin API key cannot enable step-up verification; use an admin session with TOTP enabled",
+			"STEP_UP_ADMIN_API_KEY_FORBIDDEN", nil)
+		return false
+	}
+	subject, ok := middleware.GetAuthSubjectFromContext(c)
+	if !ok || subject.UserID <= 0 {
+		response.ErrorWithDetails(c, http.StatusForbidden,
+			"Enabling step-up verification requires an authenticated admin session",
+			"STEP_UP_ENABLE_REQUIRES_TOTP", nil)
+		return false
+	}
+	if h.userService == nil {
+		response.InternalError(c, "Step-up precondition check unavailable")
+		return false
+	}
+	user, err := h.userService.GetByID(c.Request.Context(), subject.UserID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return false
+	}
+	if !user.TotpEnabled {
+		response.ErrorWithDetails(c, http.StatusBadRequest,
+			"Enable two-factor authentication (TOTP) for your account before turning on step-up verification",
+			"STEP_UP_ENABLE_REQUIRES_TOTP", nil)
+		return false
+	}
+	return true
+}
+
 func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 	var req UpdateSettingsRequest
 	if err := c.ShouldBindJSON(&req); err != nil {
@@ -353,6 +390,34 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		return
 	}
 
+	// 两个安全开关的请求字段为指针：省略字段=保持现值，避免旧客户端/脚本
+	// 用不含新字段的全量 payload 保存设置时把安全开关静默重置。
+	sessionBindingEnabled := previousSettings.SessionBindingEnabled
+	if req.SessionBindingEnabled != nil {
+		sessionBindingEnabled = *req.SessionBindingEnabled
+	}
+	stepUpEnabled := previousSettings.StepUpEnabled
+	if req.StepUpEnabled != nil {
+		stepUpEnabled = *req.StepUpEnabled
+	}
+
+	// 开启敏感操作 step-up 门控属自锁风险操作：仅允许本人已启用 TOTP 的管理员会话开启，
+	// 否则开启后操作者立即被挡在所有敏感操作之外。仅在 false→true 的开启瞬间校验，
+	// 保持开启状态的常规设置保存不受影响。
+	if stepUpEnabled && !previousSettings.StepUpEnabled {
+		if !h.ensureActorTotpForStepUp(c) {
+			return
+		}
+	}
+	// 关闭 step-up 门控本身就是敏感操作：防止拿到管理员会话的攻击者先关闸再执行导出/备份。
+	// previousSettings 已证实开关处于开启状态，使用无条件门控变体，
+	// 避免门控内部二次读取开关时因存储故障 fail-open（前端捕获 STEP_UP_REQUIRED 弹码重试）。
+	if !stepUpEnabled && previousSettings.StepUpEnabled {
+		if !middleware.EnforceStepUpAlways(c, h.totpService, h.userService) {
+			return
+		}
+	}
+
 	// 验证参数
 	if req.DefaultConcurrency < 1 {
 		req.DefaultConcurrency = 1
@@ -1181,7 +1246,8 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		FrontendURL:                      req.FrontendURL,
 		InvitationCodeEnabled:            req.InvitationCodeEnabled,
 		TotpEnabled:                      req.TotpEnabled,
-		SessionBindingEnabled:            req.SessionBindingEnabled,
+		SessionBindingEnabled:            sessionBindingEnabled,
+		StepUpEnabled:                    stepUpEnabled,
 		AuditLogRetentionDays:            req.AuditLogRetentionDays,
 		LoginAgreementEnabled:            req.LoginAgreementEnabled,
 		LoginAgreementMode:               loginAgreementMode,
@@ -1710,6 +1776,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		TotpEnabled:                                            updatedSettings.TotpEnabled,
 		TotpEncryptionKeyConfigured:                            h.settingService.IsTotpEncryptionKeyConfigured(),
 		SessionBindingEnabled:                                  updatedSettings.SessionBindingEnabled,
+		StepUpEnabled:                                          updatedSettings.StepUpEnabled,
 		AuditLogRetentionDays:                                  updatedSettings.AuditLogRetentionDays,
 		LoginAgreementEnabled:                                  updatedSettings.LoginAgreementEnabled,
 		LoginAgreementMode:                                     updatedSettings.LoginAgreementMode,

backend/internal/handler/admin/user_handler.go

@@ -33,6 +33,7 @@ type UserHandler struct {
 	billingCache          service.BillingCache                // T17/T18 缓存失效（PUT/POST 路径）
 	totpService           *service.TotpService                // 角色提升为管理员的 step-up 门控
 	userService           *service.UserService
+	settingService        *service.SettingService // step-up 功能开关
 }
 
 // NewUserHandler creates a new admin user handler
@@ -43,6 +44,7 @@ func NewUserHandler(
 	billingCache service.BillingCache,
 	totpService *service.TotpService,
 	userService *service.UserService,
+	settingService *service.SettingService,
 ) *UserHandler {
 	return &UserHandler{
 		adminService:          adminService,
@@ -51,6 +53,7 @@ func NewUserHandler(
 		billingCache:          billingCache,
 		totpService:           totpService,
 		userService:           userService,
+		settingService:        settingService,
 	}
 }
 
@@ -275,7 +278,7 @@ func (h *UserHandler) Create(c *gin.Context) {
 
 	// 创建管理员账号属权限敏感操作：需最近完成 step-up 2FA 验证。
 	if req.Role == service.RoleAdmin {
-		if !middleware.EnforceStepUp(c, h.totpService, h.userService) {
+		if !middleware.EnforceStepUp(c, h.totpService, h.userService, h.settingService) {
 			return
 		}
 	}
@@ -331,7 +334,7 @@ func (h *UserHandler) Update(c *gin.Context) {
 			return
 		}
 		if target.Role != service.RoleAdmin {
-			if !middleware.EnforceStepUp(c, h.totpService, h.userService) {
+			if !middleware.EnforceStepUp(c, h.totpService, h.userService, h.settingService) {
 				return
 			}
 		}

backend/internal/handler/admin/user_handler_activity_test.go

@@ -35,7 +35,7 @@ func TestUserHandlerListIncludesActivityFieldsAndSortParams(t *testing.T) {
 			UpdatedAt:    lastLoginAt,
 		},
 	}
-	handler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil)
+	handler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil, nil)
 
 	recorder := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(recorder)
@@ -89,7 +89,7 @@ func TestUserHandlerGetByIDIncludesActivityFields(t *testing.T) {
 			UpdatedAt:    lastLoginAt,
 		},
 	}
-	handler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil)
+	handler := NewUserHandler(adminSvc, nil, nil, nil, nil, nil, nil)
 
 	recorder := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(recorder)

backend/internal/handler/admin/user_handler_batch_limits_test.go

@@ -44,7 +44,7 @@ func (s *batchLimitsAdminServiceStub) BatchUpdateLimits(_ context.Context, userI
 func setupBatchLimitsRouter(serviceStub service.AdminService) *gin.Engine {
 	gin.SetMode(gin.TestMode)
 	router := gin.New()
-	handler := NewUserHandler(serviceStub, nil, nil, nil, nil, nil)
+	handler := NewUserHandler(serviceStub, nil, nil, nil, nil, nil, nil)
 	router.POST("/api/v1/admin/users/batch-limits", handler.BatchUpdateLimits)
 	return router
 }

backend/internal/handler/admin/user_handler_get_deleted_test.go

@@ -26,7 +26,7 @@ func (s *getByIDAdminStub) GetUserIncludeDeleted(_ context.Context, id int64) (*
 func setupGetByIDRouter(svc service.AdminService) *gin.Engine {
 	gin.SetMode(gin.TestMode)
 	r := gin.New()
-	h := NewUserHandler(svc, nil, nil, nil, nil, nil)
+	h := NewUserHandler(svc, nil, nil, nil, nil, nil, nil)
 	r.GET("/admin/users/:id", h.GetByID)
 	return r
 }

backend/internal/handler/admin/user_handler_list_apikey_group_test.go

@@ -39,7 +39,7 @@ func TestAdminUserList_ParsesAPIKeyGroupID(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			stub := &listUsersFilterStub{AdminService: newStubAdminService()}
 			r := gin.New()
-			h := NewUserHandler(stub, nil, nil, nil, nil, nil)
+			h := NewUserHandler(stub, nil, nil, nil, nil, nil, nil)
 			r.GET("/admin/users", h.List)
 
 			w := httptest.NewRecorder()

backend/internal/handler/admin/user_handler_role_stepup_test.go

@@ -29,7 +29,7 @@ func setupRoleStepUpRouter(t *testing.T) (*gin.Engine, *stubAdminService) {
 		Status: service.StatusActive,
 	})
 
-	h := NewUserHandler(adminSvc, nil, nil, nil, nil, nil)
+	h := NewUserHandler(adminSvc, nil, nil, nil, nil, nil, nil)
 	router.POST("/api/v1/admin/users", h.Create)
 	router.PUT("/api/v1/admin/users/:id", h.Update)
 	return router, adminSvc