feat(distribution): 默认结算冷却天数改为系统配置驱动

新建站点的默认冷却期原先硬编码 7 天（OnboardSite 字面量、DB 列默认、两套前端表单），
改一处要同时动三处。现统一到系统配置键 distribution.default_settlement_hold_days：

- systemconfig 注册该键（默认 7，0=立即释放，0-365 校验），纳入 admin 可见配置
- distribution 建站两条路径（CreateSite 未传值、OnboardSite 缺省）读取该配置，缺失或非法回落 7
- 新增 GET /admin/distribution/site-defaults 暴露当前有效默认值
- Vue / React 站点表单改为运行时拉取，拉取失败保持 7
- DB 列默认值保留作为兜底，站点级 settlement_hold_days 仍可单独覆盖

backend/internal/modules/systemconfig/repository.go

@@ -54,6 +54,7 @@ var defaultConfigs = []defaultConfig{
 	{Key: cooperationFeedbackConfigKey, Value: defaultCooperationFeedbackConfigValue(), Description: "首页 合作与反馈入口配置 JSON（标题/副标题/展示图片）"},
 	{Key: publishOptionsConfigKey, Value: defaultPublishOptionsConfigValue(), Description: "发布页选项配置 JSON"},
 	{Key: salePriceConfigKey, Value: defaultSalePriceConfigValue(), Description: "内部出售定价规则 JSON"},
+	{Key: DistributionSettlementHoldDaysKey, Value: defaultDistributionSettlementHoldDaysValue(), Description: "新建分销站点默认结算冷却天数（0=立即释放，最大 365）"},
 	{Key: "backup.schedule_enabled", Value: "true", Description: "在线备份定时任务是否启用"},
 	{Key: "backup.full_hour", Value: "4", Description: "每日全量备份小时（Asia/Shanghai）"},
 	{Key: "backup.full_minute", Value: "30", Description: "每日全量备份分钟（Asia/Shanghai）"},
@@ -65,6 +66,7 @@ var adminVisibleConfigKeys = []string{
 	"chat.default_support_admin_id",
 	"chat.listing_group_welcome",
 	"chat.renter_retention_days_after_order_end",
+	"distribution.default_settlement_hold_days",
 	"handoff.owner_return_confirm_timeout_minutes",
 	"handoff.owner_submit_timeout_minutes",
 	"handoff.renter_confirm_timeout_minutes",
@@ -140,6 +142,9 @@ func (r *Repository) FindValue(ctx context.Context, key string) (string, error)
 }
 
 func (r *Repository) Update(ctx context.Context, actorID uint64, key string, req UpdateRequest, meta AuditMeta) (*ConfigDTO, error) {
+	if err := validateConfigValue(key, req.Value); err != nil {
+		return nil, err
+	}
 	var row model.SystemConfig
 	err := r.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
 		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).Where("`key` = ?", key).First(&row).Error; err != nil {