feat: clean up old notifications

a new scheduled task that will delete old notifications. the default is
that over 1 year old notifications will be deleted. the scheduled task
is run every 24 hours. this can be disabled by setting the
retention period to false in the notifications config.

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

plugins/notifications-backend/config.d.ts

@@ -28,5 +28,10 @@ export interface Config {
      * Throttle duration between notification sending, defaults to 50ms
      */
     throttleInterval?: HumanDuration | string;
+    /**
+     * Time to keep the notifications in the database, defaults to 365 days.
+     * Can be disabled by setting to false.
+     */
+    retention?: HumanDuration | string | false;
   };
 }