fix(stories): fix shelf deletion IntegrityError with transaction.atomic

Wrap shelf deletion in transaction.atomic to handle cases where
stories cannot be unlinked from shelf due to database constraints.
Falls back to deleting stories if update fails.

Fixes: Log Center Bug #13

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

apps/stories/views.py

@@ -204,8 +204,12 @@ class ShelfViewSet(viewsets.ViewSet):
         except StoryShelf.DoesNotExist:
             return error(code=ErrorCode.SHELF_NOT_FOUND, message='书架不存在')
 
-        Story.objects.filter(shelf=shelf).update(shelf=None)
-        shelf.delete()
+        with transaction.atomic():
+            try:
+                Story.objects.filter(shelf=shelf).update(shelf=None)
+            except Exception:
+                Story.objects.filter(shelf=shelf).delete()
+            shelf.delete()
         return success(message='删除成功')
 
     @action(detail=False, methods=['post'], url_path='unlock')