feat(backup): full DR bundle export + admin-configurable offsite destinations

Backend-agnostic disaster-recovery backup engine that runs on the current
storage backend (no storage cutover required):

- Full-bundle export: db.dump (pg_dump custom) + every storage blob +
  manifest.json with per-object SHA-256, streamed as a tar. Entry points:
  admin UI download, GET /api/v1/admin/backup/export, scripts/create-full-backup.ts.
- Admin-configurable push destinations (backup_destinations table, migration
  0091): SFTP/SSH, S3-compatible (reuses the minio client), and mounted
  path/NAS behind one transport interface (test/push/prune). Secrets AES-GCM
  at rest; API returns only *IsSet markers.
- Opt-in per-destination AES-256 bundle encryption (scrypt KDF, streamed) +
  scripts/decrypt-backup.ts for restore.
- Wired the previously-dead database-backup cron to runScheduledBackupPush
  (push to enabled destinations, prune to retention, alert super-admins on
  failure).

Tests: 1608 unit/integration pass; tsc + lint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -65,3 +65,6 @@ tmp/
 # Internal docs + Claude instructions: kept local-only, not in the shared repo
 docs/
 /CLAUDE.md
+
+# Client-facing feature screenshots (real PII — do not commit)
+docs/feature-screenshots/