feat: add scheduler and mail outbox

SMTP adapter with TLS and a fake, outbox with retries, backoff, rate limit and restart safety, scheduler tick with job-run records started by gunicorn, and a send_test_mail command.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

docker/entrypoint.sh

@@ -4,6 +4,7 @@ set -eu
 python manage.py migrate --noinput
 
 exec gunicorn portal.config.wsgi:application \
+    --config /app/docker/gunicorn.conf.py \
     --bind 0.0.0.0:8000 \
     --workers 1 \
     --threads 4 \