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_test.go

@@ -649,6 +649,23 @@ func TestDistributionBrandingAndMarkup(t *testing.T) {
 		t.Fatalf("unexpected branding: %+v", updated)
 	}
 
+	qrURL := "/api/files/object?key=qrcode/brand-site.png"
+	qrUpdated, err := repo.UpdateSite(t.Context(), 0, site.ID, UpdateSiteRequest{ListingQrcodeURL: &qrURL})
+	if err != nil {
+		t.Fatalf("UpdateSite listing qrcode failed: %v", err)
+	}
+	if qrUpdated.ListingQrcodeURL != qrURL {
+		t.Fatalf("listing_qrcode_url = %q, want %q", qrUpdated.ListingQrcodeURL, qrURL)
+	}
+	cleared := ""
+	qrCleared, err := repo.UpdateSite(t.Context(), 0, site.ID, UpdateSiteRequest{ListingQrcodeURL: &cleared})
+	if err != nil {
+		t.Fatalf("clear listing qrcode failed: %v", err)
+	}
+	if qrCleared.ListingQrcodeURL != "" {
+		t.Fatalf("cleared listing_qrcode_url = %q, want empty", qrCleared.ListingQrcodeURL)
+	}
+
 	// Verify PublicSiteInfo contains branding
 	pubInfo, err := repo.GetPublicSiteInfo(t.Context(), "brand_site", "")
 	if err != nil {