feat: add Backend Mode toggle to disable user self-service

Add a system-wide "Backend Mode" that disables user self-registration
and self-service while keeping admin panel and API gateway fully
functional. When enabled, only admin can log in; all user-facing
routes return 403.

Backend:
- New setting key `backend_mode_enabled` with atomic cached reads (60s TTL)
- BackendModeUserGuard middleware blocks non-admin authenticated routes
- BackendModeAuthGuard middleware blocks registration/password-reset auth routes
- Login/Login2FA/RefreshToken handlers reject non-admin when enabled
- TokenPairWithUser struct for role-aware token refresh
- 20 unit tests (middleware + service layer)

Frontend:
- Router guards redirect unauthenticated users to /login
- Admin toggle in Settings page
- Login page hides register link and footer in backend mode
- 9 unit tests for router guard logic
- i18n support (en/zh)

27 files changed, 833 insertions(+), 17 deletions(-)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/admin/setting_handler.go

@@ -125,6 +125,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		OpsMetricsIntervalSeconds:            settings.OpsMetricsIntervalSeconds,
 		MinClaudeCodeVersion:                 settings.MinClaudeCodeVersion,
 		AllowUngroupedKeyScheduling:          settings.AllowUngroupedKeyScheduling,
+		BackendModeEnabled:                   settings.BackendModeEnabled,
 	})
 }
 
@@ -199,6 +200,9 @@ type UpdateSettingsRequest struct {
 
 	// 分组隔离
 	AllowUngroupedKeyScheduling bool `json:"allow_ungrouped_key_scheduling"`
+
+	// Backend Mode
+	BackendModeEnabled bool `json:"backend_mode_enabled"`
 }
 
 // UpdateSettings 更新系统设置
@@ -473,6 +477,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		IdentityPatchPrompt:              req.IdentityPatchPrompt,
 		MinClaudeCodeVersion:             req.MinClaudeCodeVersion,
 		AllowUngroupedKeyScheduling:      req.AllowUngroupedKeyScheduling,
+		BackendModeEnabled:               req.BackendModeEnabled,
 		OpsMonitoringEnabled: func() bool {
 			if req.OpsMonitoringEnabled != nil {
 				return *req.OpsMonitoringEnabled
@@ -571,6 +576,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		OpsMetricsIntervalSeconds:            updatedSettings.OpsMetricsIntervalSeconds,
 		MinClaudeCodeVersion:                 updatedSettings.MinClaudeCodeVersion,
 		AllowUngroupedKeyScheduling:          updatedSettings.AllowUngroupedKeyScheduling,
+		BackendModeEnabled:                   updatedSettings.BackendModeEnabled,
 	})
 }
 
@@ -725,6 +731,9 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.AllowUngroupedKeyScheduling != after.AllowUngroupedKeyScheduling {
 		changed = append(changed, "allow_ungrouped_key_scheduling")
 	}
+	if before.BackendModeEnabled != after.BackendModeEnabled {
+		changed = append(changed, "backend_mode_enabled")
+	}
 	if before.PurchaseSubscriptionEnabled != after.PurchaseSubscriptionEnabled {
 		changed = append(changed, "purchase_subscription_enabled")
 	}