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/encrypt_fields.html

@@ -0,0 +1,23 @@
+{{/* Password fields shared by the full-backup and domain-export forms. The
+     checkbox is the switch: unticked, the download keeps its historic plain
+     form (.tar.gz / .json); ticked, the file is sealed in a password-encrypted
+     envelope (.spbk / .spde). The fields start hidden and are revealed by
+     panel.js — with JavaScript blocked they are simply always visible, and the
+     server still decides from the checkbox alone. */}}
+{{define "encryptfields"}}
+<div class="encrypt">
+  <label class="check">
+    <input type="checkbox" name="encrypt" value="1" data-encrypt-toggle>
+    <span>Encrypt with a password</span>
+  </label>
+  <div class="encrypt-fields" data-encrypt-fields>
+    <label for="encpw">Password</label>
+    <input id="encpw" name="password" type="password" autocomplete="new-password"
+           minlength="{{.MinPwLen}}" placeholder="at least {{.MinPwLen}} characters">
+    <label for="encpw2">Repeat password</label>
+    <input id="encpw2" name="password_confirm" type="password" autocomplete="new-password">
+    <p class="muted">Keep this password: without it the file cannot be opened,
+    and SelfPost does not store it anywhere.</p>
+  </div>
+</div>
+{{end}}