Sends one queued email (see MishkaCmsCore.Mail), on the mail queue.
The job carries the delivery's id, its site, its idempotency key and the email sealed with the
endpoint's key — a reset or sign-in email holds a token, and finished jobs stay in oban_jobs
for a week. It carries no provider key: the provider reads its own when the job runs.
A failure the provider calls worth retrying (MishkaCmsCore.Mail.Provider.retry?/1) is retried
with Oban's backoff, with the same idempotency key; anything else, and the last attempt, ends
the delivery :failed. A site that lost its provider in the meantime ends it :not_configured.
Once the provider has answered, the site's history older than mailer.history_days goes
(MishkaCmsCore.Mail.prune/1).