feat: admin audit logging across all admin actions

- AdminLogger: JSON-lines → /var/log/xamxam.log (prod) / storage/logs/admin.log (dev)
  + best-effort DB mirror to admin_audit_log table
- DB: admin_audit_log table, share_links.is_archived column
- ShareLink: archive() replaces delete(), toggleActive() returns new state,
  listActive()/listArchived() split, validateLink blocks archived slugs
- All action handlers wired: publish, unpublish, visibility, delete, csv/db export,
  tfe add/edit, tags, pages, apropos, form-help, access-request, maintenance,
  settings (formulaire toggles, objet types, smtp update), smtp-test
- acces.php: archive button replaces delete; collapsible archived links section
- setup-server.sh: provision /var/log/xamxam.log (www-data:xamxam 640)

app/public/admin/actions/form-help.php

@@ -18,6 +18,7 @@ $key     = $_POST['form_help_key'] ?? '';
 $content = $_POST['content']       ?? '';
 
 require_once APP_ROOT . '/src/Database.php';
+require_once APP_ROOT . '/src/AdminLogger.php';
 $db = new Database();
 
 if (!in_array($key, Database::FORM_HELP_KEYS, true)) {
@@ -28,6 +29,7 @@ if (!in_array($key, Database::FORM_HELP_KEYS, true)) {
 
 try {
     $db->setFormHelpBlock($key, $content);
+    AdminLogger::make()->logFormStructureEdit($key);
     App::flash('success', 'Bloc « ' . htmlspecialchars($key) . ' » mis à jour.');
 } catch (Exception $e) {
     error_log('form-help save error: ' . $e->getMessage());