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/available_channel_handler.go

@@ -21,19 +21,30 @@ import (
 type AvailableChannelHandler struct {
 	channelService *service.ChannelService
 	apiKeyService  *service.APIKeyService
+	settingService *service.SettingService
 }
 
 // NewAvailableChannelHandler 创建用户侧可用渠道 handler。
 func NewAvailableChannelHandler(
 	channelService *service.ChannelService,
 	apiKeyService *service.APIKeyService,
+	settingService *service.SettingService,
 ) *AvailableChannelHandler {
 	return &AvailableChannelHandler{
 		channelService: channelService,
 		apiKeyService:  apiKeyService,
+		settingService: settingService,
 	}
 }
 
+// featureEnabled 返回 available-channels 开关是否启用。默认关闭（opt-in）。
+func (h *AvailableChannelHandler) featureEnabled(c *gin.Context) bool {
+	if h.settingService == nil {
+		return false
+	}
+	return h.settingService.GetAvailableChannelsRuntime(c.Request.Context()).Enabled
+}
+
 // userAvailableGroup 用户可见的分组概要（白名单字段）。
 type userAvailableGroup struct {
 	ID       int64  `json:"id"`
@@ -89,6 +100,13 @@ func (h *AvailableChannelHandler) List(c *gin.Context) {
 		return
 	}
 
+	// Feature 未启用时返回空数组（不暴露渠道信息）。检查放在认证之后，
+	// 保持与未开关前的 401 行为一致：未登录先 401，登录后再按开关决定。
+	if !h.featureEnabled(c) {
+		response.Success(c, []userAvailableChannel{})
+		return
+	}
+
 	userGroups, err := h.apiKeyService.GetAvailableGroups(c.Request.Context(), subject.UserID)
 	if err != nil {
 		response.ErrorFrom(c, err)