feat: add LDAP authentication settings and toggle

Add an admin-only LDAP configuration UI with an enable toggle and full
sign-in integration.

Backend:
- ldapSettings model + migration (single-row config)
- GET/PUT/test routes under /ldap (admin-gated; bind password masked)
- shared ldapClient with RFC 4515 filter escaping and empty-password guard
- signin tries local auth first, then LDAP when enabled (find-or-create
  local user) so the bootstrap admin is never locked out

Frontend:
- LDAP settings page (Switch + form + test connection) under /admin/ldap
- AdminNav tabs between user management and LDAP
- ldapControl util, types, and Ldap i18n namespace for all 6 locales

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app/[locale]/admin/page.tsx

@@ -17,6 +17,7 @@ export default function Page({ params }: PageType) {
   const t = useTranslations('Admin');
   const messages: AdminMessages = {
     userManagement: t('user_management'),
+    ldap: t('ldap'),
     avatar: t('avatar'),
     id: t('id'),
     email: t('email'),