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/server/api_contract_test.go

@@ -988,6 +988,7 @@ func TestAPIContracts(t *testing.T) {
 					"channel_monitor_enabled": true,
 					"channel_monitor_mode": "v1",
 					"channel_monitor_hide_throughput": true,
+					"channel_monitor_show_quota": false,
 					"channel_monitor_default_interval_seconds": 60,
 					"available_channels_enabled": false,
 					"model_plaza_enabled": false,
@@ -1298,6 +1299,7 @@ func TestAPIContracts(t *testing.T) {
 					"channel_monitor_enabled": true,
 					"channel_monitor_mode": "v1",
 					"channel_monitor_hide_throughput": true,
+					"channel_monitor_show_quota": false,
 					"channel_monitor_default_interval_seconds": 60,
 					"available_channels_enabled": false,
 					"model_plaza_enabled": false,