Mars backup-lane inventory for fleet registry + fix backup-db.sh EXPECTED_REF fallthrough

Fleet DB-hardening step 5 (pmmaster-evolutiva-cc ruling 2026-08-07): after step 1 lands, inventory Mars's backup lanes (cron/timer, script, destination, retention) in the same shape as coder-venus-cc's venus writeup, for pmmaster-evolutiva-cc's fleet backup registry. Most of this is already documented in mars-backup.md (evolutiva-backup@mars.timer on whey, owned by os-whey-cc-w, /gdrive/backup/evolutiva/mars, 30-day retention) — verify it's still accurate rather than re-deriving. Also folded in: mars/db/backup-db.sh has an EXPECTED_REF gap — falls through to whatever DATABASE_URL_DIRECT happens to be exported if .env.local is missing, instead of failing loud. Fix that as part of this WI (Class-S, no authz touch).