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

Dockerfile

@@ -27,6 +27,7 @@ RUN SECRET_KEY=build-time-only-value-not-used-at-runtime-000000 \
     LDAP_CA_FILE=/app/README.md LDAP_PUBLISHER_GROUP_DN=build LDAP_ADMIN_GROUP_DN=build \
     OIDC_ISSUER=https://build.invalid/ OIDC_CLIENT_ID=build OIDC_CLIENT_SECRET=build \
     PUBLIC_BASE_URL=https://build.invalid \
+    SMTP_HOST=build.invalid SMTP_USERNAME=build SMTP_PASSWORD=build MAIL_FROM=build@build.invalid \
     sh -c '/app/.venv/bin/python manage.py compilemessages && /app/.venv/bin/python manage.py collectstatic --noinput'
 
 # Not part of the runtime image: `docker compose --profile test run --rm live-test`