Block bot registrations and add cron cleanup for unconfirmed accounts

register.php was a fully open signup form with no bot defenses — the
likely source of the unconfirmed accounts piling up in admin/users.php.

- Add an always-on honeypot field + timing trap to register.php: either
  tripping silently pretends success without creating an account, so a
  bot doesn't learn it was caught. No configuration needed.

- Add optional Google reCAPTCHA v3 support (includes/recaptcha.php,
  recaptcha_enabled()/verify_recaptcha(), no Composer dependency — a
  raw file_get_contents() POST like mailer.php's SMTP socket approach).
  A failed check here shows a real, visible error instead of the silent
  honeypot path, since a legitimate low-score user deserves a retry.

- Configure it through admin/settings.php's new "Bot Protection" section,
  mirroring the existing SMTP pattern exactly: recaptcha_enabled/
  recaptcha_site_key/recaptcha_secret_key in site_settings, secret key
  masked the same way smtp_pass now is (blank submission keeps it
  unchanged). install.php seeds sane defaults so the feature stays off
  until explicitly configured — fully backward compatible.

- Add cron/cleanup_unconfirmed.php: deletes accounts still unconfirmed
  after 3 days. CLI-only (refuses to run over HTTP, and cron/.htaccess
  denies web access to the directory as a second layer) since it's an
  unattended, irreversible deletion. Safe by construction — login.php
  already refuses login to unconfirmed accounts, so these rows can never
  own a session/novena_group/custom_prayer row. Not wired up
  automatically; README documents the Hostinger cron job to schedule it.

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

cron/.htaccess

@@ -0,0 +1,5 @@
+# Defense in depth: cron/cleanup_unconfirmed.php already refuses to run
+# outside the CLI, but block web access to this directory entirely too.
+Require all denied
+Order allow,deny
+Deny from all