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/backup.html

@@ -10,8 +10,11 @@
   start. TLS certificates and the mail queue are not included.</p>
   <p class="muted"><strong>The backup file is a secret</strong> (it contains
   private keys and credentials). Store and transfer it securely and delete it
-  once the restore succeeds.</p>
-  <form class="inline" method="post" action="/backup">
+  once the restore succeeds. Encrypting it below is the simplest way to do that:
+  the download is then a <code>.spbk</code> file that only the password opens.</p>
+  {{if .BackupErr}}<p class="error">{{.BackupErr}}</p>{{end}}
+  <form method="post" action="/backup">
+    {{template "encryptfields" .}}
     <button type="submit">Download full backup</button>
   </form>
 </div>
@@ -25,7 +28,17 @@
   {{if .ImportErr}}<p class="error">{{.ImportErr}}</p>{{end}}
   <form method="post" action="/domains/import" enctype="multipart/form-data">
     <label for="importfile">Domain export file</label>
-    <input id="importfile" name="file" type="file" accept=".json,application/json" required>
+    <input id="importfile" name="file" type="file" accept=".json,.spde,application/json" required>
+    <div class="encrypt">
+      <label class="check">
+        <input type="checkbox" data-encrypt-toggle>
+        <span>The file is encrypted (<code>.spde</code>)</span>
+      </label>
+      <div class="encrypt-fields" data-encrypt-fields>
+        <label for="importpw">Password</label>
+        <input id="importpw" name="import_password" type="password" autocomplete="off">
+      </div>
+    </div>
     <button type="submit">Import domain</button>
   </form>
 </div>