feat(api): add VW commercial vehicle support, fix BOM leaf category handling

Add WV1/WV2/WV3 WMI codes for Volkswagen commercial vehicles (Ticari)
mapped to vn_parts service. Skip BOM links in category traversal since
they are leaf categories returning parts, not subgroups.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/categories/categories.service.ts

@@ -107,6 +107,11 @@ export class CategoriesService {
       return [];
     }
 
+    // BOM links are leaf categories — they return parts, not subgroups
+    if (linkPath.includes("/bom/")) {
+      return [];
+    }
+
     const subGroups = await this.pl24Service.fetchSubGroupsByPath(
       linkPath,
       catalogInfo.serviceName,