Delivers one push message to one token. One job per token is deliberate: FCM v1 and APNs are one-request-per-token anyway, and it buys exact per-token Oban retry/backoff — a half-delivered batch job would re-push duplicates on retry.
Fires after_push_sent on every terminal outcome (delivered, token
disabled, permanent failure, or retries exhausted) — never on a transient
error that will retry.