Push notifications

• GET/push/devices
  List push devicesList the push devices the authenticated user registered through this app, newest first. Disabled devices are excluded.
• POST/push/devices
  Register a push deviceStore an FCM registration token so the user can receive push notifications for new chat messages. Registration is idempotent on token, so re-sending the same token does not create duplicates. If the token already belongs to another user (device hand-off or account switch), ownership moves to the authenticated user. A previously disabled token is re-enabled.
• DELETE/push/devices/:deviceId
  Unregister a push deviceStop push notifications for the given device. Call this on sign-out. Returns 404 for devices registered by another app or owned by another user.