fix(electron-backend): process zero-delay database cancellation (#1295)

.changes/electron-backend-zero-delay-cancellation.md

@@ -0,0 +1,8 @@
+---
+type: fix
+area: electron-backend
+---
+
+Cancelling a large Xtream import now stops database work even while it is
+running at full speed, instead of letting the import finish before processing
+the cancel request.