feat(library): add atomic bulk assign-books action to AdminAuthorViewSet

- Add assign-books action to AdminAuthorViewSet for bulk adding and removing book associations in a single SQL query

apps/library/views_admin.py

@@ -135,6 +135,23 @@ class AdminAuthorViewSet(ModelViewSet):
 
         return queryset.order_by("full_name", "id")
 
+    @action(detail=True, methods=["post"], url_path="assign-books")
+    def assign_books(self, request, pk=None):
+        author = self.get_object()
+        add_ids = request.data.get("add_ids", []) or request.data.get("book_ids", [])
+        remove_ids = request.data.get("remove_ids", [])
+
+        if add_ids:
+            Book.objects.filter(id__in=add_ids).update(author=author)
+
+        if remove_ids:
+            Book.objects.filter(id__in=remove_ids, author=author).update(author=None)
+
+        return Response({
+            "status": "success",
+            "attached_count": author.books.count()
+        })
+
 
 class AdminBookViewSet(ModelViewSet):
     permission_classes = [IsAuthenticated, IsSuperAdmin]