feat(channels): gate available channels behind feature switch (backend)

Add a DB-backed soft switch "available_channels_enabled" controlling
the user-facing /channels/available endpoint and sidebar entry. Default
to false (opt-in) — the feature stays invisible until an admin enables
it under Admin Settings > Features.

- domain_constants: SettingKeyAvailableChannelsEnabled
- settings_view: AllSettings/PublicSettings + AvailableChannelsEnabled
- setting_service: public+all read/write, seed default "false",
  GetAvailableChannelsRuntime helper (fail-closed on read error)
- admin setting_handler: UpdateSettingsRequest *bool + update branch
  + audit diff entry
- public setting_handler: expose via GET /api/v1/settings
- available_channel_handler: featureEnabled() guard — returns empty
  list after auth when disabled (401 precedes the feature check to
  preserve existing behavior)

backend/internal/service/domain_constants.go

@@ -254,6 +254,11 @@ const (
 	// pre-filled when creating a new channel monitor from the admin UI. Range: [15, 3600].
 	SettingKeyChannelMonitorDefaultIntervalSeconds = "channel_monitor_default_interval_seconds"
 
+	// SettingKeyAvailableChannelsEnabled is a DB-backed soft switch for the "Available Channels"
+	// user-facing aggregate view. When false: user endpoint returns an empty list and the
+	// sidebar entry is hidden. Defaults to false (opt-in feature).
+	SettingKeyAvailableChannelsEnabled = "available_channels_enabled"
+
 	// =========================
 	// Overload Cooldown (529)
 	// =========================