feat(distribution): 支持分站发布群固定二维码

主站分销配置可为每个站点绑定可复用企微码，号主在该站上传账号后发到对应发布群，不再消耗主站码池。

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

backend/internal/modules/distribution/repository.go

@@ -253,6 +253,9 @@ func (r *Repository) CreateSite(ctx context.Context, req CreateSiteRequest) (*Si
 	if req.DefaultSupportGroupID != nil {
 		row.DefaultSupportGroupID = req.DefaultSupportGroupID
 	}
+	if req.ListingQrcodeURL != nil {
+		row.ListingQrcodeURL = strings.TrimSpace(*req.ListingQrcodeURL)
+	}
 	if req.TaxEnabled != nil {
 		row.TaxEnabled = *req.TaxEnabled
 	}
@@ -305,6 +308,9 @@ func (r *Repository) UpdateSite(ctx context.Context, adminID, id uint64, req Upd
 		if req.DefaultSupportGroupID != nil {
 			row.DefaultSupportGroupID = req.DefaultSupportGroupID
 		}
+		if req.ListingQrcodeURL != nil {
+			row.ListingQrcodeURL = strings.TrimSpace(*req.ListingQrcodeURL)
+		}
 		if req.TaxEnabled != nil {
 			row.TaxEnabled = *req.TaxEnabled
 		}
@@ -467,6 +473,7 @@ func siteDTO(row model.DistributionSite) SiteDTO {
 		Status:                row.Status,
 		DisplayAccountEnabled: row.DisplayAccountEnabled,
 		DefaultSupportGroupID: row.DefaultSupportGroupID,
+		ListingQrcodeURL:      row.ListingQrcodeURL,
 		TaxEnabled:            row.TaxEnabled,
 		TaxRateBPS:            row.TaxRateBPS,
 		SettlementHoldDays:    row.SettlementHoldDays,