Support push notifications to multiple devices per user in MAS Mobile (not just the last-used device)

Today MAS Mobile push notifications only reach one device per user - whichever device the user opened most recently. MAS keeps a single push registration per user (one device token and token type), and every time the app is opened or resumed on a device, that device re-registers its token and overwrites the one stored before it. When a notification is sent, the service delivers it to that single stored token, so only the last-used device receives it.

The result is that a user who is signed in on more than one device - for example an iPhone and an Android tablet - only gets notifications on whichever one they opened last. 

The switch happens silently, with no error or warning, and every other device simply stops receiving pushes until the user opens the app on it again.

We would like MAS Mobile to support delivering push notifications to all of a user's active registered devices instead of just one. This means keeping a separate registration for each device rather than overwriting a single record, and sending each notification to every one of the user's active.

This matters because technicians often use more than one device, and time-critical alerts such as newly assigned or emergency work orders need to reach them reliably regardless of which device they are holding. Delivering to only the last-used device causes missed and delayed notifications, and it differs from the behavior people expect from everyday apps like email and messaging, where every device a user owns receives the notification.