fix: prefetch invoice items for faster calculation

Author: nijel

weblate_web/crm/views.py

@@ -588,7 +588,9 @@ def _get_invoices_and_totals(
         self, year: int, month: int | None = None
     ) -> tuple[list[Invoice], dict[UUID, Decimal]]:
         """Fetch invoices and pre-calculate totals (shared helper)."""
-        query = Invoice.objects.filter(kind=InvoiceKind.INVOICE, issue_date__year=year)
+        query = Invoice.objects.filter(
+            kind=InvoiceKind.INVOICE, issue_date__year=year
+        ).prefetch_related("invoiceitem_set")
         if month:
             query = query.filter(issue_date__month=month)