Require deviceId on POST /notifications/register, upsert by deviceId
while preserving lastNotifiedAt and internal id, prune duplicate token
rows, migrate legacy fcmToken-keyed JSON, and add register logs.
Extend StoredRow and Device with deviceId; resolve pushes by scanning
fcmToken.
Assign stable ids to stored tokens, migrate legacy ISO lastNotifiedAt
to epoch ms, replace setLastNotifiedAt with db.update, and persist
lastNotifiedAt only after a successful FCM send. Extend Device with
optional lastNotifiedAt (ms).
