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/settings_view.go

@@ -129,6 +129,9 @@ type SystemSettings struct {
 	ChannelMonitorEnabled                 bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds  int  `json:"channel_monitor_default_interval_seconds"`
 
+	// Available Channels feature (user-facing aggregate view)
+	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
+
 	// Claude Code version check
 	MinClaudeCodeVersion string
 	MaxClaudeCodeVersion string
@@ -217,6 +220,9 @@ type PublicSettings struct {
 	// Channel Monitor feature
 	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
+
+	// Available Channels feature (user-facing aggregate view)
+	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
 }
 
 type WeChatConnectOAuthConfig struct {