feat(api): signal loadError when a PL24 catalog fetch fails

A category whose PL24 drill/parts fetch errors (e.g. a broken catalog snapshot
returning HTTP 500) was indistinguishable from a genuinely empty leaf — both
came back with empty parts, so the UI showed a misleading "no parts found".

getCategoryWithParts now: (1) for a pl24 group node (link_wid marks a groups
table) with no children, drills via getChildren — returns the subgroups if PL24
serves them, otherwise flags loadError (these group nodes are never legitimately
empty); (2) sets loadError when the leaf parts fetch throws. loadError is only
true when the lists are actually empty, so genuine empty leaves stay clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -670,7 +670,29 @@ export class CategoriesService {
       // to the parts path rather than returning a misleading empty leaf.
     }
 
+    // PL24 main/sub-group node (its link_wid marks a groups table) with no
+    // children yet — drill it. If PL24 serves subgroups, return them as a
+    // parent. If the drill comes back empty, the catalog snapshot is broken
+    // server-side: surface that as a load error instead of a misleading
+    // "no parts" leaf (these group nodes are never legitimately empty).
+    if (category.source === "pl24" && category.linkWid?.includes("Group") && category.vehicleId) {
+      const groupChildren = await this.getChildren(categoryId);
+      const base = {
+        id: category.id,
+        name: category.name,
+        description: category.nameOriginal || null,
+        parentId: category.parentId || null,
+        parts: [],
+        schemaPics: [],
+        hotspots: [],
+      };
+      return groupChildren.length > 0
+        ? { ...base, children: groupChildren }
+        : { ...base, loadError: true };
+    }
+
     // Leaf category — get or fetch parts
+    let loadError = false;
     let discoveredChildren: any[] = [];
     let dbParts = await this.db.select().from(parts).where(eq(parts.categoryId, categoryId));
 
@@ -1090,6 +1112,10 @@ export class CategoriesService {
               }
             }
           } catch (err) {
+            // PL24 fetch failed (e.g. broken catalog snapshot → HTTP 500).
+            // Signal a load error so the UI can offer a retry instead of
+            // rendering a misleading empty "no parts" leaf.
+            loadError = true;
             this.logger.error(
               `Failed to fetch parts for category ${categoryId}: ${(err as Error).message}`,
             );
@@ -1172,6 +1198,9 @@ export class CategoriesService {
       parts: dbParts,
       schemaPics: mappedPics,
       hotspots: mappedHotspots,
+      // Only meaningful when the lists are empty: true means a source fetch
+      // failed (retryable), as opposed to a genuinely empty leaf.
+      loadError: loadError && dbParts.length === 0 && mappedPics.length === 0,
     };
   }