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

frontend/src/api/admin/settings.ts

@@ -469,6 +469,10 @@ export interface SystemSettings {
   balance_low_notify_recharge_url: string;
   account_quota_notify_enabled: boolean;
   account_quota_notify_emails: NotifyEmailEntry[];
+
+  // Channel Monitor feature switch
+  channel_monitor_enabled: boolean;
+  channel_monitor_default_interval_seconds: number;
 }
 
 export interface UpdateSettingsRequest {
@@ -618,6 +622,10 @@ export interface UpdateSettingsRequest {
   balance_low_notify_recharge_url?: string;
   account_quota_notify_enabled?: boolean;
   account_quota_notify_emails?: NotifyEmailEntry[];
+
+  // Channel Monitor feature switch
+  channel_monitor_enabled?: boolean;
+  channel_monitor_default_interval_seconds?: number;
 }
 
 /**