[P3] backup: encryption at rest + offsite (age/gpg) and offsite retention (currently unbounded)

[gfargo]

Idea

Encrypt backups at rest and offsite, and implement offsite retention.

Why

Backups are plaintext SQL/dumps of full production data; offsite sync passes no SSE flags and there's no client-side encryption. BACKUP_OFFSITE_RETENTION_DAYS is documented and printed but no code prunes remote objects (unbounded cost).

Scope

• Optional age/gpg client-side encryption for local + offsite artifacts.
• SSE flags on aws s3/b2 uploads; post-upload checksum verification.
• Implement offsite retention pruning.

Acceptance

Backups can be encrypted at rest and offsite; offsite retention actually deletes aged remote objects; uploads are integrity-checked.

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Filed from the 2026-07-03 full-codebase audit (docs/AUDIT-2026-07.md).