feat: optional password encryption for backup and domain export (code-review.md § Phase 1.5)

Both secret-bearing downloads can now be sealed with a password. Unticked, the
forms produce exactly the files they did before.

- internal/secretfile: envelope format — magic/type/scrypt params/salt/nonce
  prefix header, then 64 KiB AES-256-GCM chunks each authenticated with the
  header, its counter and an end-of-stream flag, so truncation, reordering and
  tampering fail to open instead of restoring a plausible prefix. Streams both
  ways, so a full backup never sits in memory.
- Panel: "Encrypt with a password" checkbox on the full-backup and
  domain-export forms (shared partial, toggled from panel.js — no inline
  script); domain import detects an encrypted export by magic bytes, not by
  extension, and asks for the password.
- selfpost-backup: writes .spbk when given a password and converts one back
  with -decrypt, which a restore needs. The password comes from
  SELFPOST_BACKUP_PASSWORD or -password-file, never argv.
- Docs: README, security.md (+ accepted risk: encryption stays opt-in),
  architecture.md, progress.md, CHANGELOG.

Verified locally: panel-encrypted archive decrypts through the CLI and unpacks;
wrong password and password mismatch are refused; UI checked in a browser.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/architecture.md

@@ -176,6 +176,15 @@ Not in `/data`: TLS certificates (reverse-proxy mount), Postfix queue
 `/data` tree; version check on restore. Stopped-container `tar` of `./data` is
 safe (see README).
 
+**Optional encryption** of the two secret-bearing downloads
+([internal/secretfile](../internal/secretfile/secretfile.go)): password →
+scrypt → AES-256-GCM over 64 KiB chunks, each authenticated with the header,
+its counter and an end-of-stream flag (so truncation and reordering fail to
+open). Full backup `.tar.gz` → `.spbk`, domain export `.json` → `.spde`; the
+plain forms remain the default. Domain import detects the envelope by magic
+bytes; an encrypted full backup is converted back with `selfpost-backup
+-decrypt` before restore.
+
 ---
 
 ## Security (summary)