feat(backup): age at-rest encryption of bundles (CONTRACT_004 §4.3)

Close the known gap: backup bundles were uploaded unencrypted, relying
solely on destination access control. Now every data artifact is
age-encrypted on the VM before upload and decrypted on restore.

- backup-remote.sh: assemble rustfs blobs into rustfs-blobs.tar.zst (so the
  whole bundle is one encrypted unit), then age -r <recipient> each artifact
  to <name>.age and drop the plaintext. MANIFEST.json stays cleartext — it
  is the inventory + integrity gate and carries no secrets; it records each
  artifact's PLAINTEXT sha256 so restore verifies after decrypt.
- restore-remote.sh: materialise the age identity to a 0600 file, decrypt
  each .age, then run the existing sha + scratch-restore asserts; add a
  rustfs-blobs extract+assert.
- backup.sh / restore.sh: pass the public recipient (arg) / secret identity
  (stdin, never argv) from passphrase-encrypted config.
- provision/index.ts: install age + zstd on the VM via cloud-init so a fresh
  DR VM (T13) has the backup tools from first boot.
- Pulumi.foundation.yaml: seed backup.ageRecipient (public) + backup.ageIdentity
  (secure:). The identity lives in config so {repo + passphrase} can decrypt a
  bundle even after total Vault loss (CONTRACT_004 §4.3).

Validated live: encrypted backup + restore-verify PASS from both RustFS and
offsite; bucket shows only *.age + cleartext MANIFEST.json.

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

bootstrap/Pulumi.foundation.yaml

@@ -63,4 +63,7 @@ config:
     secure: v1:9YpTkFoQanMwxAQV:dJ4YmXS0aOTHPbuK1H6AJ0SAJ0CjYX0iIyLOQAUNfsOWLsSy5TXxPpGecieBWkzc4AALDkJNlQN9Xo6Q0ZcaSg==
   vaultCredentials:rootToken:
     secure: v1:OUpYMjnaftxMUKjv:2m+dydQopXGRleeX6ddhYSHgHP7HHZXYLAvQHXUvaA91qajoxU+VugDB/Rs=
+  foundation:backup.ageRecipient: age1x6dmgtt2eahpvyzkmy6j80rts28chw2lcam0rcxq3nhc8ld649sslzpsy4
+  foundation:backup.ageIdentity:
+    secure: v1:VCFVXswrmMrXyFbr:p4pfG/Kp2lreetYX4O86rZqpU1xQugRycF+PBBiNGZnaD0c15R+mJuLNrl0rBXY5vJwyZTbNSpFY1zPQ7TwuQcVp9h8oiGcgVEobsbb4BBp3lFhsObllgYM9
 encryptionsalt: v1:5YhUt8BVfH0=:v1:DPCHl+7zwn4RaMPj:A19tZzBlZ1NmDtTWrHreEKk5e8idyw==