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,