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/i18n/locales/en.ts

@@ -4530,6 +4530,7 @@ export default {
       description: 'Manage registration, email verification, default values, and SMTP settings',
       tabs: {
         general: 'General',
+        features: 'Feature Switches',
         security: 'Security',
         users: 'Users',
         gateway: 'Gateway',
@@ -4537,6 +4538,16 @@ export default {
         backup: 'Backup',
         payment: 'Payment',
       },
+      features: {
+        channelMonitor: {
+          title: 'Channel Monitor',
+          description: 'Periodically probe configured channels and surface availability / latency to users. Turning it off stops the scheduler and returns an empty list on the user page.',
+          enabled: 'Enable Channel Monitor',
+          enabledHint: 'Disabling stops background checks; existing history is preserved.',
+          defaultInterval: 'Default check interval (seconds)',
+          defaultIntervalHint: 'Pre-fills the interval when creating a new monitor; each monitor can override it. Range 15 – 3600.',
+        },
+      },
       emailTabDisabledTitle: 'Email Verification Not Enabled',
       emailTabDisabledHint: 'Enable email verification in the Security tab to configure SMTP settings.',
       registration: {