feat: automate postgres backups to b2

[GitAddRemote]

Closes #125

Summary

• add production backup and restore scripts, nightly cron/logrotate bootstrap wiring, and a deployment runbook section for backup verification and restore
• write rclone.conf during production deploys and run a pre-deploy PostgreSQL backup before the backend rollout
• update infra tests, secrets docs, and CI/CD docs to match the new off-site backup contract

Testing

• git -C /tmp/station-issue-125 diff --check
• node /tmp/station-issue-125/infra/tests/infrastructure.test.mjs

Notes

• #125 is now In Progress on the Station Kanban Board
• the backup flow uses the current production compose contract (docker compose ... exec -T postgres) instead of hardcoding a container name
• production will need valid B2_ACCOUNT_ID, B2_APPLICATION_KEY, and B2_BUCKET secrets plus a real B2 bucket before the new pre-deploy backup step can succeed