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
 }

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)

backend/internal/handler/dto/settings.go

@@ -187,6 +187,9 @@ type SystemSettings 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 aggregate view)
+	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
 }
 
 type DefaultSubscriptionSetting struct {
@@ -237,6 +240,8 @@ type PublicSettings struct {
 
 	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
+
+	AvailableChannelsEnabled bool `json:"available_channels_enabled"`
 }
 
 // OverloadCooldownSettings 529过载冷却配置 DTO

backend/internal/handler/setting_handler.go

@@ -73,5 +73,7 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 
 		ChannelMonitorEnabled:                settings.ChannelMonitorEnabled,
 		ChannelMonitorDefaultIntervalSeconds: settings.ChannelMonitorDefaultIntervalSeconds,
+
+		AvailableChannelsEnabled: settings.AvailableChannelsEnabled,
 	})
 }