feat(cli): cmd_backup_export — pack vault into .relbak

Reads the vault layout from disk, prompts for backup passphrase
(zxcvbn-gated, independent of the live vault key), tars .git/
unless --no-history, optionally bundles the reference JPEG, and
atomic-writes the .relbak. Leaves .relicario/last_backup marker
for cmd_status.

Cargo.lock

@@ -1594,6 +1594,7 @@ dependencies = [
  "rpassword",
  "serde",
  "serde_json",
+ "tar",
  "tempfile",
  "url",
  "zeroize",