feat(distribution): 分站可下调收号比例且不超过主站上限

让分销站能按自身能力下调收号比例扩大利润，同时把主站回收比例作为硬边界，避免分站把比例加得比主站更高。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/modules/listing/site_receive.go

@@ -0,0 +1,46 @@
+package listing
+
+import (
+	"context"
+	"strings"
+
+	"hfb_sys/backend/internal/model"
+)
+
+func (r *Repository) ReceiveRatioDeltaBySiteCode(ctx context.Context, siteCode string) float64 {
+	siteCode = strings.TrimSpace(siteCode)
+	if r == nil || r.db == nil || siteCode == "" {
+		return 0
+	}
+	var delta float64
+	_ = r.db.WithContext(ctx).Model(&model.DistributionSite{}).
+		Select("COALESCE(receive_ratio_delta, 0)").
+		Where("site_code = ? AND status = 'active'", siteCode).
+		Limit(1).
+		Scan(&delta)
+	return NormalizeSiteReceiveRatioDelta(delta)
+}
+
+func (r *Repository) ResolveReceiveRatioDelta(ctx context.Context, listingID uint64, siteCode string) (float64, bool) {
+	if code := strings.TrimSpace(siteCode); code != "" {
+		return r.ReceiveRatioDeltaBySiteCode(ctx, code), true
+	}
+	if r == nil || r.db == nil || listingID == 0 {
+		return 0, false
+	}
+	var siteID *uint64
+	if err := r.db.WithContext(ctx).Model(&model.RentalListing{}).
+		Select("distribution_site_id").
+		Where("id = ?", listingID).
+		Limit(1).
+		Scan(&siteID).Error; err != nil || siteID == nil || *siteID == 0 {
+		return 0, false
+	}
+	var delta float64
+	_ = r.db.WithContext(ctx).Model(&model.DistributionSite{}).
+		Select("COALESCE(receive_ratio_delta, 0)").
+		Where("id = ? AND status = 'active'", *siteID).
+		Limit(1).
+		Scan(&delta)
+	return NormalizeSiteReceiveRatioDelta(delta), true
+}