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/handler/admin/setting_handler.go

@@ -238,6 +238,8 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 
 		ChannelMonitorEnabled:                settings.ChannelMonitorEnabled,
 		ChannelMonitorDefaultIntervalSeconds: settings.ChannelMonitorDefaultIntervalSeconds,
+
+		AvailableChannelsEnabled: settings.AvailableChannelsEnabled,
 	}
 	response.Success(c, systemSettingsResponseData(payload, authSourceDefaults))
 }
@@ -432,6 +434,9 @@ type UpdateSettingsRequest struct {
 	// Channel Monitor feature switch
 	ChannelMonitorEnabled                *bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds *int  `json:"channel_monitor_default_interval_seconds"`
+
+	// Available Channels feature switch (user-facing)
+	AvailableChannelsEnabled *bool `json:"available_channels_enabled"`
 }
 
 // UpdateSettings 更新系统设置
@@ -1238,6 +1243,12 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.ChannelMonitorDefaultIntervalSeconds
 		}(),
+		AvailableChannelsEnabled: func() bool {
+			if req.AvailableChannelsEnabled != nil {
+				return *req.AvailableChannelsEnabled
+			}
+			return previousSettings.AvailableChannelsEnabled
+		}(),
 	}
 
 	authSourceDefaults := &service.AuthSourceDefaultSettings{
@@ -1471,6 +1482,8 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 
 		ChannelMonitorEnabled:                updatedSettings.ChannelMonitorEnabled,
 		ChannelMonitorDefaultIntervalSeconds: updatedSettings.ChannelMonitorDefaultIntervalSeconds,
+
+		AvailableChannelsEnabled: updatedSettings.AvailableChannelsEnabled,
 	}
 	response.Success(c, systemSettingsResponseData(payload, updatedAuthSourceDefaults))
 }
@@ -1833,6 +1846,9 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.ChannelMonitorDefaultIntervalSeconds != after.ChannelMonitorDefaultIntervalSeconds {
 		changed = append(changed, "channel_monitor_default_interval_seconds")
 	}
+	if before.AvailableChannelsEnabled != after.AvailableChannelsEnabled {
+		changed = append(changed, "available_channels_enabled")
+	}
 	changed = appendAuthSourceDefaultChanges(changed, beforeAuthSourceDefaults, afterAuthSourceDefaults)
 	return changed
 }