Gotify

Gotify is a lightweight server that delivers push notifications to your phone, tablet, or desktop from any application able to call a simple HTTP endpoint. Messages are grouped into topics and applications, access is controlled with per-application tokens, and the official Android app plus a browser client let you read and dismiss alerts from anywhere.

Teams use it for build results, server alerts, backup jobs, and home automation events instead of a hosted push service.

Because the server lives on your own hardware, notification content stays under your control, there is no subscription or message quota to pay for, and the database and TLS setup are yours to back up and restore.