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/distribution/repository.go

@@ -13,6 +13,7 @@ import (
 	"hfb_sys/backend/internal/auditlog"
 	"hfb_sys/backend/internal/model"
 	listingpkg "hfb_sys/backend/internal/modules/listing"
+	"hfb_sys/backend/internal/modules/systemconfig"
 	"hfb_sys/backend/internal/modules/wallet"
 
 	"github.com/redis/go-redis/v9"
@@ -220,6 +221,23 @@ func (r *Repository) FindSite(ctx context.Context, adminID, id uint64) (*SiteDTO
 	return &item, nil
 }
 
+// resolveDefaultSettlementHoldDays 读取系统配置里的新建站点默认结算冷却天数，缺失或异常时回落到内置默认值。
+func (r *Repository) resolveDefaultSettlementHoldDays(ctx context.Context) int {
+	if r == nil || r.db == nil {
+		return systemconfig.DefaultDistributionSettlementHoldDays
+	}
+	var row model.SystemConfig
+	if err := r.db.WithContext(ctx).Where("`key` = ?", systemconfig.DistributionSettlementHoldDaysKey).First(&row).Error; err != nil {
+		return systemconfig.DefaultDistributionSettlementHoldDays
+	}
+	return systemconfig.ParseDistributionSettlementHoldDays(row.Value)
+}
+
+// DefaultSettlementHoldDays 暴露当前有效的默认结算冷却天数，供前端表单使用。
+func (r *Repository) DefaultSettlementHoldDays(ctx context.Context) int {
+	return r.resolveDefaultSettlementHoldDays(ctx)
+}
+
 func (r *Repository) CreateSite(ctx context.Context, req CreateSiteRequest) (*SiteDTO, error) {
 	row := model.DistributionSite{
 		SiteCode: strings.TrimSpace(req.SiteCode), SiteName: strings.TrimSpace(req.SiteName), Domain: strings.TrimSpace(req.Domain),
@@ -243,6 +261,8 @@ func (r *Repository) CreateSite(ctx context.Context, req CreateSiteRequest) (*Si
 			return nil, err
 		}
 		row.SettlementHoldDays = holdDays
+	} else {
+		row.SettlementHoldDays = r.resolveDefaultSettlementHoldDays(ctx)
 	}
 	if err := r.db.WithContext(ctx).Create(&row).Error; err != nil {
 		return nil, err
@@ -1404,7 +1424,7 @@ func (r *Repository) OnboardSite(ctx context.Context, req OnboardSiteRequest) (*
 		if req.TaxRateBPS != nil {
 			taxRate = *req.TaxRateBPS
 		}
-		holdDays := 7
+		holdDays := r.resolveDefaultSettlementHoldDays(ctx)
 		if req.SettlementHoldDays != nil {
 			normalized, err := normalizeSettlementHoldDays(*req.SettlementHoldDays)
 			if err != nil {