deliver_scheduled_emails: Use a queue, instead of infinite retries.

`deliver_scheduled_emails` tries to deliver the email synchronously,
and if it fails, it retries after 10 seconds.  Since it does not track
retries, and always tries the earliest-scheduled-but-due message
first, the worker will not make forward progress if there is a
persistent failure with that message, and will retry indefinitely.
This can result in excessive network or email delivery charges from
the remote SMTP server.

Switch to delivering emails via a new queue worker.  The
`deliver_scheduled_emails` job now serves only to pull deferred jobs
out of the table once they are due, insert them into RabbitMQ, and
then delete them.  This limits the potential for head-of-queue
failures to failures inserting into RabbitMQ, which is more reasonable
than failures speaking to a complex external system we do not control.
Retries and any connections to the SMTP server are left to the
RabbitMQ consumer.

We build a new RabbitMQ queue, rather than use the existing
`email_senders` queue, because that queue is expected to be reasonably
low-latency, for things like missed message notifications.  The
`send_future_email` codepath which inserts into ScheduledEmails is
also (ab)used to digest emails, which are extremely bursty in their
frequency -- and a large burst could significantly delay emails behind
it in the queue.

The new queue is explicitly only for messages which were not initiated
by user actions (e.g., invitation reminders, digests, new account
follow-ups) which are thus not latency-sensitive.

Fixes: #32463.

scripts/lib/check_rabbitmq_queue.py

@@ -14,6 +14,7 @@ from scripts.lib.zulip_tools import atomic_nagios_write, get_config, get_config_
 
 normal_queues = [
     "deferred_work",
+    "deferred_email_senders",
     "digest_emails",
     "email_mirror",
     "email_senders",
@@ -49,7 +50,7 @@ MAX_SECONDS_TO_CLEAR: defaultdict[str, int] = defaultdict(
     digest_emails=1200,
     missedmessage_mobile_notifications=120,
     embed_links=60,
-    email_senders=240,
+    deferred_email_senders=240,
 )
 CRITICAL_SECONDS_TO_CLEAR: defaultdict[str, int] = defaultdict(
     lambda: 60,
@@ -57,7 +58,7 @@ CRITICAL_SECONDS_TO_CLEAR: defaultdict[str, int] = defaultdict(
     missedmessage_mobile_notifications=180,
     digest_emails=1800,
     embed_links=90,
-    email_senders=300,
+    deferred_email_senders=300,
 )