Retain a single production backup

[ewlarson]

Summary

• reduce the shared production backup retention default from three copies to one
• align PostgreSQL, Elasticsearch, cron, Kamal, and example configuration defaults
• assert the single-copy production default in deployment and backup tests

Context

The Kithe Bridge database is currently canonical, and retaining multiple roughly 90 GB local dumps unnecessarily pressures the filesystem shared with Elasticsearch.

Related to #383.

Verification

• python -m pytest backend/tests/scripts/test_backup_postgres_to_s3.py backend/tests/test_kamal_deploy_config.py
• make lint-check