Deliverable 1: ops/backup.sh and tradein-mvp/deploy/backup-tradein-db.sh now
write a sentinel file on every verified-good run. A new ops/check-backup-
staleness.sh (separate cron entry, hourly) alerts via the existing Telegram
channel (same TELEGRAM_BOT_TOKEN/TELEGRAM_CHAT_ID idiom as ops/uptime-
healthcheck.sh, transition-tracked so it doesn't spam) if a sentinel goes
stale. Shared logic (notify/sentinel/state) factored into ops/lib-backup.sh
so it isn't triplicated; the two existing scripts' own hardening (integrity
checks, retention, etc.) is untouched.
Deliverable 2: ops/backup-forgejo.sh — Forgejo (git.gendsgn.ru) has no backup
today. Dumps the shared-postgres `forgejo` DB + tars the bare-repo tree, both
off-box to s3://gendsgn-backups/forgejo/ under a SEPARATE, narrower S3 key
(root-of-bucket writer key stays out of this). The key doesn't exist yet —
the script refuses to run and exits non-zero, loudly, until the four
FORGEJO_S3_* vars are filled in (see the example env file and PR description
for the exact bucket policy JSON to create it with).
Refs #2203, #2989
