feat(channel-monitor): add feature switch settings + fix extra_models save

Settings:
- New "功能开关" tab between 通用设置 and 安全与认证
- ChannelMonitorEnabled toggle: runner skips scheduling when false,
  user-facing list returns empty
- ChannelMonitorDefaultIntervalSeconds (15-3600): pre-fills interval
  when creating a new monitor; each monitor can still override

Bug fix:
- ModelTagInput now commits pending input on blur, not just Enter/Tab.
  Previously clicking "save" with an un-Enter'd extra model would drop
  the value (DB stored extra_models=[] even when user typed entries).

Backend:
- domain_constants: SettingKeyChannelMonitor{Enabled,DefaultIntervalSeconds}
- SettingService.GetChannelMonitorRuntime: lightweight getter used by
  runner tick + user handler per-request (fail-open on DB error)
- Runner tickDueChecks: bails early when feature disabled
- ChannelMonitorUserHandler: checks feature flag before serving
- Comment on runner doc: scheduler state is implicit (every tick re-reads
  ListEnabled from DB), so CRUD ops on monitors self-maintain the schedule

Bump VERSION to 0.1.114.25

backend/internal/handler/dto/settings.go

@@ -183,6 +183,10 @@ type SystemSettings struct {
 	BalanceLowNotifyRechargeURL string             `json:"balance_low_notify_recharge_url"`
 	AccountQuotaNotifyEnabled   bool               `json:"account_quota_notify_enabled"`
 	AccountQuotaNotifyEmails    []NotifyEmailEntry `json:"account_quota_notify_emails"`
+
+	// Channel Monitor feature switch
+	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
+	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
 }
 
 type DefaultSubscriptionSetting struct {
@@ -230,6 +234,9 @@ type PublicSettings struct {
 	AccountQuotaNotifyEnabled        bool             `json:"account_quota_notify_enabled"`
 	BalanceLowNotifyThreshold        float64          `json:"balance_low_notify_threshold"`
 	BalanceLowNotifyRechargeURL      string           `json:"balance_low_notify_recharge_url"`
+
+	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
+	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
 }
 
 // OverloadCooldownSettings 529过载冷却配置 DTO