Fix N+1 queries in API (#5684)

* Fix N+1 query in API quotas list

* fix membership N+1

* fix vouchers N+1 budget_used

* rename and reuse Voucher.annotate_budget_used_orders to budget_used

* fix flake8

src/pretix/api/views/voucher.py

@@ -19,6 +19,7 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
+
 from django.db import transaction
 from django.db.models import F, Q
 from django.utils.timezone import now
@@ -64,8 +65,13 @@ class VoucherViewSet(viewsets.ModelViewSet):
     permission = 'can_view_vouchers'
     write_permission = 'can_change_vouchers'
 
+    @scopes_disabled()  # we have an event check here, and we can save some performance on subqueries
     def get_queryset(self):
-        return self.request.event.vouchers.select_related('seat').all()
+        return Voucher.annotate_budget_used(
+            self.request.event.vouchers
+        ).select_related(
+            'item', 'quota', 'seat', 'variation'
+        )
 
     @transaction.atomic()
     def create(self, request, *args, **kwargs):