fix(catalog): guard empty subgroup insert after nav-crumb filter (500)

When fetchSubGroupsByPath returns only nav-crumb stubs (e.g. a "Portal"-only
page for a Hyundai BODY node), the post-filter set is empty and drizzle
.values([]) threw → HTTP 500. Compute `unique` unconditionally and only
insert when non-empty; otherwise fall through to the leaf/parts path for a
clean empty node.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/api/src/catalog/catalog.service.ts

@@ -785,15 +785,19 @@ export class CatalogService {
         gearbox,
       );
 
-      if (subGroups.length > 0) {
-        const seenNames = new Set<string>();
-        const unique = subGroups.filter((sg) => {
-          if (this.isNavCrumbLink(sg.linkPath)) return false;
-          if (seenNames.has(sg.name)) return false;
-          seenNames.add(sg.name);
-          return true;
-        });
+      // After dropping nav-crumb junk the subgroup set can be empty (upstream
+      // returned only a "Portal"/external stub for this node). Guard the insert —
+      // drizzle .values([]) throws — and fall through to the leaf/parts path,
+      // which yields a clean empty node instead of a 500.
+      const seenNames = new Set<string>();
+      const unique = subGroups.filter((sg) => {
+        if (this.isNavCrumbLink(sg.linkPath)) return false;
+        if (seenNames.has(sg.name)) return false;
+        seenNames.add(sg.name);
+        return true;
+      });
 
+      if (unique.length > 0) {
         const insertData = unique.map((sg) => ({
           catalogVehicleId,
           vehicleId: null as string | null,