[Notifications & Event Service] Adjust push notification registration event

[mplorentz]

In PushNotificationService.registerForNotifications(...) we send a kind 6666 to our push notification service. This kind was just a random one we picked, but now it is in use by NIP-90. We should change the kind number to 30078 which is "custom app data" as defined by NIP-78.

This isn't causing any bugs right now, but it will prevent confusion and potentially bugs in the future.

[mplorentz]

To be clear: this needs to be changed on the server side and iOS app simultaneously. In fact we should have the server continue accepting kind 6666 for a week or so after deploying the iOS part of this so that our users have time to update.