feat: admin tag management, maintenance mode, TFE visibility states

Tags admin:
- Database: getAllTagsWithCount(), renameTag(), mergeTag(), deleteTag()
- public/admin/tags.php: table with inline rename/merge/delete forms, CSRF-guarded
- public/admin/actions/tag.php: routes on action=rename|merge|delete
- templates/admin/head.php: 'Mots-clés' nav link
- admin.css: admin-inline-form, admin-btn--sm/warning/danger variants

Maintenance mode:
- config/bootstrap.php: gate on MAINTENANCE_FLAG file; admin/ and maintenance.php exempt
- public/maintenance.php: 503 dark minimal page
- public/admin/actions/maintenance.php: enable/disable toggle
- public/admin/index.php: status bar with toggle button
- admin.css: admin-maintenance-bar styles

TFE Visibility (Libre/Interne/Interdit via existing access_type_id):
- migration 002_add_visibility.sql: seeds access_types if missing
- Database: setVisibility(), bulkSetVisibility(), getAccessTypes()
- public/media.php: blocks thesis files for access_type_id=3
- public/tfe.php: shows access_type, context_note; hides file panel for Interdit
- public/admin/edit.php: access_type_id select + context_note textarea; saves both
- public/admin/index.php: three-state badge (Libre/Interne/Interdit) per row
- public/admin/actions/visibility.php: single + bulk visibility action handler
- admin.css: status-access badge variants

public/admin/actions/maintenance.php

@@ -0,0 +1,28 @@
+<?php
+require_once __DIR__ . '/../../../config/bootstrap.php';
+require_once __DIR__ . '/../../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+if ($_SERVER['REQUEST_METHOD'] !== 'POST'
+    || !isset($_POST['csrf_token'], $_SESSION['csrf_token'])
+    || !hash_equals($_SESSION['csrf_token'], $_POST['csrf_token'])) {
+    http_response_code(403);
+    die("Accès refusé.");
+}
+
+$action = $_POST['action'] ?? '';
+
+if ($action === 'enable_maintenance') {
+    file_put_contents(MAINTENANCE_FLAG, date('c'));
+    $_SESSION['success'] = "Mode maintenance activé.";
+} elseif ($action === 'disable_maintenance') {
+    if (file_exists(MAINTENANCE_FLAG)) {
+        unlink(MAINTENANCE_FLAG);
+    }
+    $_SESSION['success'] = "Mode maintenance désactivé.";
+} else {
+    $_SESSION['error'] = "Action inconnue.";
+}
+
+header('Location: /admin/');
+exit();