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/validate.go

@@ -15,6 +15,12 @@ const (
 	maxUsernameLen = 64
 )
 
+// minSecretFilePasswordLen is the floor for the password protecting an
+// encrypted backup or domain export. Such a file is offline and can be attacked
+// at leisure, so the floor matches the administrator password's rather than the
+// weaker "any password is better than none".
+const minSecretFilePasswordLen = minAdminPasswordLen
+
 // validateUsername enforces a strict server-side whitelist (spec 7.6.2):
 // letters, digits, dot, dash, underscore. Client validation is never trusted.
 func validateUsername(u string) error {