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/settlement_default_test.go

@@ -0,0 +1,54 @@
+package distribution
+
+import (
+	"testing"
+
+	"hfb_sys/backend/internal/model"
+	"hfb_sys/backend/internal/modules/systemconfig"
+)
+
+// TestCreateSiteUsesConfiguredDefaultSettlementHoldDays 验证新建站点未显式传值时读取系统配置的默认冷却天数。
+func TestCreateSiteUsesConfiguredDefaultSettlementHoldDays(t *testing.T) {
+	db := distributionTestDB(t)
+	repo := NewRepository(db)
+	if err := db.Create(&model.SystemConfig{
+		Key:   systemconfig.DistributionSettlementHoldDaysKey,
+		Value: "3",
+	}).Error; err != nil {
+		t.Fatal(err)
+	}
+	dto, err := repo.CreateSite(t.Context(), CreateSiteRequest{SiteCode: "cfg_hold", SiteName: "配置冷却站", Domain: "cfghold.dist.com"})
+	if err != nil {
+		t.Fatalf("CreateSite err = %v", err)
+	}
+	if dto.SettlementHoldDays != 3 {
+		t.Fatalf("DTO settlement_hold_days = %d, want 3", dto.SettlementHoldDays)
+	}
+	var site model.DistributionSite
+	if err := db.Where("site_code = ?", "cfg_hold").First(&site).Error; err != nil {
+		t.Fatal(err)
+	}
+	if site.SettlementHoldDays != 3 {
+		t.Fatalf("DB settlement_hold_days = %d, want 3", site.SettlementHoldDays)
+	}
+}
+
+// TestCreateSiteFallsBackWhenConfigMissing 验证系统配置缺失时回落到内置默认 7 天。
+func TestCreateSiteFallsBackWhenConfigMissing(t *testing.T) {
+	db := distributionTestDB(t)
+	repo := NewRepository(db)
+	dto, err := repo.CreateSite(t.Context(), CreateSiteRequest{SiteCode: "no_cfg_hold", SiteName: "兜底冷却站", Domain: "nocfghold.dist.com"})
+	if err != nil {
+		t.Fatalf("CreateSite err = %v", err)
+	}
+	if dto.SettlementHoldDays != systemconfig.DefaultDistributionSettlementHoldDays {
+		t.Fatalf("DTO settlement_hold_days = %d, want %d", dto.SettlementHoldDays, systemconfig.DefaultDistributionSettlementHoldDays)
+	}
+	var site model.DistributionSite
+	if err := db.Where("site_code = ?", "no_cfg_hold").First(&site).Error; err != nil {
+		t.Fatal(err)
+	}
+	if site.SettlementHoldDays != systemconfig.DefaultDistributionSettlementHoldDays {
+		t.Fatalf("DB settlement_hold_days = %d, want %d", site.SettlementHoldDays, systemconfig.DefaultDistributionSettlementHoldDays)
+	}
+}