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/service/domain_constants.go

@@ -242,6 +242,18 @@ const (
 	// SettingKeyOpsRuntimeLogConfig stores JSON config for runtime log settings.
 	SettingKeyOpsRuntimeLogConfig = "ops_runtime_log_config"
 
+	// =========================
+	// Channel Monitor (渠道监控)
+	// =========================
+
+	// SettingKeyChannelMonitorEnabled is a DB-backed soft switch for the channel monitor feature.
+	// When false: runner skips scheduling and user-facing endpoints return an empty list.
+	SettingKeyChannelMonitorEnabled = "channel_monitor_enabled"
+
+	// SettingKeyChannelMonitorDefaultIntervalSeconds controls the default interval (seconds)
+	// pre-filled when creating a new channel monitor from the admin UI. Range: [15, 3600].
+	SettingKeyChannelMonitorDefaultIntervalSeconds = "channel_monitor_default_interval_seconds"
+
 	// =========================
 	// Overload Cooldown (529)
 	// =========================