Use local Postgres backups in production

[ewlarson]

Summary

• add a local Postgres backup target with compressed dumps, manifests, retention pruning, and a process lock to skip overlapping runs
• configure production to use mounted local backup storage for Postgres while keeping S3 settings available for non-local targets
• lower production LOG_LEVEL to INFO and update the cron/Makefile wording away from Postgres S3-only backups
• configure SSR AppSignal to ignore the known Writable closed before stream finished notice in production via APPSIGNAL_FRONTEND_IGNORE_ERRORS

Validation

• make lint-check
• BTAA_SKIP_TEST_DB=1 python -m pytest backend/tests/scripts/test_backup_postgres_to_s3.py backend/tests/test_kamal_deploy_config.py
• node --check frontend/appsignal.cjs

Notes

• Elasticsearch snapshots remain S3-backed.
• Unrelated frontend working-tree changes were intentionally left out of this branch.