Add an opt-in to download the full backup as an encrypted archive instead
of a plaintext JSON file. The user supplies a passphrase over the request;
the server derives a 256-bit key with Argon2id and seals the export bytes
with AES-256-GCM into a self-describing HLX1 archive (magic, version, KDF
params, salt, iv, tag, ciphertext), mirroring the off-host backup envelope.
POST /api/export/encrypted returns the binary archive and is step-up gated
for accounts with a second factor; single-factor accounts use a normal
session. The passphrase is never stored and never logged, and there is no
server-side recovery — the UI states this. A standalone decryptor
(scripts/decrypt-export.ts) opens the archive with the passphrase outside
the app. The plaintext full-backup route now shares one payload builder so
both variants emit the same restore-compatible shape.
