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/messages/en.json

@@ -149,6 +149,7 @@
   },
   "Admin": {
     "user_management": "User Management",
+    "ldap": "LDAP",
     "avatar": "Avatar",
     "id": "ID",
     "email": "Email",
@@ -434,5 +435,27 @@
     "tag_error_create": "Failed to create tag. Please try again.",
     "tag_error_update": "Failed to update tag. Please try again.",
     "tag_error_delete": "Failed to delete tag. Please try again."
+  },
+  "Ldap": {
+    "ldap_settings": "LDAP Settings",
+    "ldap_description": "Authenticate users against an LDAP directory.",
+    "enable_ldap": "Enable LDAP",
+    "enable_ldap_desc": "Allow users to sign in with their directory credentials.",
+    "server_url": "Server URL",
+    "bind_dn": "Bind DN",
+    "bind_credentials": "Bind Password",
+    "bind_credentials_placeholder": "Leave blank to keep current password",
+    "search_base": "Search Base",
+    "search_filter": "Search Filter",
+    "email_attribute": "Email Attribute",
+    "username_attribute": "Username Attribute",
+    "save": "Save",
+    "saved": "LDAP settings saved",
+    "save_error": "Failed to save LDAP settings",
+    "test_connection": "Test Connection",
+    "test_username": "Test Username",
+    "test_password": "Test Password",
+    "test_success": "Connection successful",
+    "test_failed": "Connection failed"
   }
 }