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>

internal/web/templates/domain_detail.html

@@ -283,8 +283,11 @@
   import the DNS record stays the same, so no DNS change is needed.</p>
   <p class="muted"><strong>The export file is a secret</strong> — it contains the
   private DKIM key and application passwords. Transfer it securely and delete it
-  after the import.</p>
-  <form class="inline" method="post" action="/domains/{{.Domain.ID}}/export">
+  after the import, or encrypt it below and move a <code>.spde</code> file
+  instead; the import form asks for the password.</p>
+  {{if .ExportErr}}<p class="error">{{.ExportErr}}</p>{{end}}
+  <form method="post" action="/domains/{{.Domain.ID}}/export">
+    {{template "encryptfields" .}}
     <button type="submit">Export domain</button>
   </form>
 </div>