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/setting_service.go

@@ -452,6 +452,7 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		SettingKeyAccountQuotaNotifyEnabled,
 		SettingKeyChannelMonitorEnabled,
 		SettingKeyChannelMonitorDefaultIntervalSeconds,
+		SettingKeyAvailableChannelsEnabled,
 	}
 
 	settings, err := s.settingRepo.GetMultiple(ctx, keys)
@@ -537,6 +538,8 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 
 		ChannelMonitorEnabled:                !isFalseSettingValue(settings[SettingKeyChannelMonitorEnabled]),
 		ChannelMonitorDefaultIntervalSeconds: parseChannelMonitorInterval(settings[SettingKeyChannelMonitorDefaultIntervalSeconds]),
+
+		AvailableChannelsEnabled: settings[SettingKeyAvailableChannelsEnabled] == "true",
 	}, nil
 }
 
@@ -595,6 +598,25 @@ func (s *SettingService) GetChannelMonitorRuntime(ctx context.Context) ChannelMo
 	}
 }
 
+// AvailableChannelsRuntime is the lightweight view of the available-channels feature
+// switch consumed by the user-facing handler.
+type AvailableChannelsRuntime struct {
+	Enabled bool
+}
+
+// GetAvailableChannelsRuntime reads the available-channels feature switch directly
+// from the settings store. Fail-closed: on error returns Enabled=false, matching
+// the opt-in default (unknown ↔ disabled).
+func (s *SettingService) GetAvailableChannelsRuntime(ctx context.Context) AvailableChannelsRuntime {
+	vals, err := s.settingRepo.GetMultiple(ctx, []string{SettingKeyAvailableChannelsEnabled})
+	if err != nil {
+		return AvailableChannelsRuntime{Enabled: false}
+	}
+	return AvailableChannelsRuntime{
+		Enabled: vals[SettingKeyAvailableChannelsEnabled] == "true",
+	}
+}
+
 // SetOnUpdateCallback sets a callback function to be called when settings are updated
 // This is used for cache invalidation (e.g., HTML cache in frontend server)
 func (s *SettingService) SetOnUpdateCallback(callback func()) {
@@ -1151,6 +1173,9 @@ func (s *SettingService) buildSystemSettingsUpdates(ctx context.Context, setting
 		updates[SettingKeyChannelMonitorDefaultIntervalSeconds] = strconv.Itoa(v)
 	}
 
+	// Available channels feature switch
+	updates[SettingKeyAvailableChannelsEnabled] = strconv.FormatBool(settings.AvailableChannelsEnabled)
+
 	// Claude Code version check
 	updates[SettingKeyMinClaudeCodeVersion] = settings.MinClaudeCodeVersion
 	updates[SettingKeyMaxClaudeCodeVersion] = settings.MaxClaudeCodeVersion
@@ -1700,6 +1725,9 @@ func (s *SettingService) InitializeDefaultSettings(ctx context.Context) error {
 		SettingKeyChannelMonitorEnabled:                "true",
 		SettingKeyChannelMonitorDefaultIntervalSeconds: "60",
 
+		// Available channels feature (default disabled; opt-in)
+		SettingKeyAvailableChannelsEnabled: "false",
+
 		// Claude Code version check (default: empty = disabled)
 		SettingKeyMinClaudeCodeVersion: "",
 		SettingKeyMaxClaudeCodeVersion: "",
@@ -2008,6 +2036,9 @@ func (s *SettingService) parseSettings(settings map[string]string) *SystemSettin
 		settings[SettingKeyChannelMonitorDefaultIntervalSeconds],
 	)
 
+	// Available channels feature (default: disabled; strict true)
+	result.AvailableChannelsEnabled = settings[SettingKeyAvailableChannelsEnabled] == "true"
+
 	// Claude Code version check
 	result.MinClaudeCodeVersion = settings[SettingKeyMinClaudeCodeVersion]
 	result.MaxClaudeCodeVersion = settings[SettingKeyMaxClaudeCodeVersion]