[alarm] Include type='alarm' in Alarm.to_dict().

This is for compatibility with other backend/frontend components who
expect valid entities on events or responses, and hence may break if
`type` is not specified.

platypush/plugins/alarm/_model.py

@@ -329,6 +329,7 @@ class Alarm:
     def to_dict(self) -> dict:
         return {
             'id': self.name,
+            'type': 'alarm',
             'name': self.name,
             'when': self.when,
             'next_run': self.get_next(),