fix(dlc): 更新暗影哥特、黑色上衣与浪漫天命的商品编码

- 同步上游 DLC (聚鑫源) 最新 GoodID 映射：
  - suit_shadow_gothic: 6a13aa55e3bce430aca29b01
  - top_black_elite_trainer: 6a13aaeae3bce430aca29b03
  - suit_romantic_destiny: 6a13ab21e3bce430aca29b04
- 更新 delivery_helpers 缺省兜底映射表与 sync_product_images 常量
- 更新 provider_test 单元测试断言与 docs/商品目录.md 文档
- 验证方式：go test 单元测试通过，go vet 校验通过

backend/internal/service/provider_test.go

@@ -52,7 +52,7 @@ func TestParseProductRoutingConfig(t *testing.T) {
 	if cfg5.RoutingMode != RoutingModePriority {
 		t.Fatalf("expected priority for dual-capable legacy config, got %s", cfg5.RoutingMode)
 	}
-	if cfg5.DLCGoodID != "682ef39ca8f40c4234c59f38" {
+	if cfg5.DLCGoodID != "6a13ab21e3bce430aca29b04" {
 		t.Fatalf("expected builtin DLC good id, got %s", cfg5.DLCGoodID)
 	}