feat(settings): channel_monitor_show_quota public setting (default off)

- 新增公开设置 key（迁移 226 已插入默认 false），控制用户端监控页
  是否展示配额/余额；管理端不受影响
- 解析 fail-closed：仅字面 "true" 视为开启（对齐 available_channels_enabled
  语义，而非 hide_throughput 的 fail-open）
- 全链路贯通：domain key 常量 → setting_public（公开读取 + ChannelMonitorRuntime
  .ShowQuota + 注入 payload）→ settings_view/parse/update → handler DTO
  （admin/user 响应 + admin 更新请求）→ api_contract_test 两个 wantJSON 块

backend/internal/handler/admin/setting_handler.go

@@ -374,6 +374,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		ChannelMonitorMode:                   settings.ChannelMonitorMode,
 		ChannelMonitorDefaultIntervalSeconds: settings.ChannelMonitorDefaultIntervalSeconds,
 		ChannelMonitorHideThroughput:         settings.ChannelMonitorHideThroughput,
+		ChannelMonitorShowQuota:              settings.ChannelMonitorShowQuota,
 
 		GrokDefaultTextModel:           settings.GrokDefaultTextModel,
 		GrokCrossClientModelMapEnabled: settings.GrokCrossClientModelMapEnabled,

backend/internal/handler/admin/setting_handler_update.go

@@ -332,6 +332,7 @@ type UpdateSettingsRequest struct {
 	ChannelMonitorMode                   *string `json:"channel_monitor_mode"`
 	ChannelMonitorDefaultIntervalSeconds *int    `json:"channel_monitor_default_interval_seconds"`
 	ChannelMonitorHideThroughput         *bool   `json:"channel_monitor_hide_throughput"`
+	ChannelMonitorShowQuota              *bool   `json:"channel_monitor_show_quota"`
 
 	// Grok model mapping policy
 	GrokDefaultTextModel           *string `json:"grok_default_text_model"`
@@ -1889,6 +1890,12 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.ChannelMonitorHideThroughput
 		}(),
+		ChannelMonitorShowQuota: func() bool {
+			if req.ChannelMonitorShowQuota != nil {
+				return *req.ChannelMonitorShowQuota
+			}
+			return previousSettings.ChannelMonitorShowQuota
+		}(),
 		GrokDefaultTextModel: func() string {
 			if req.GrokDefaultTextModel != nil {
 				return *req.GrokDefaultTextModel
@@ -2342,6 +2349,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		ChannelMonitorMode:                   updatedSettings.ChannelMonitorMode,
 		ChannelMonitorDefaultIntervalSeconds: updatedSettings.ChannelMonitorDefaultIntervalSeconds,
 		ChannelMonitorHideThroughput:         updatedSettings.ChannelMonitorHideThroughput,
+		ChannelMonitorShowQuota:              updatedSettings.ChannelMonitorShowQuota,
 
 		GrokDefaultTextModel:           updatedSettings.GrokDefaultTextModel,
 		GrokCrossClientModelMapEnabled: updatedSettings.GrokCrossClientModelMapEnabled,