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

templates/admin/head.php

@@ -28,6 +28,7 @@
         <a href="/admin/add.php" class="admin-nav__link <?= $currentPage === 'add.php' ? 'active' : '' ?>">Ajouter un TFE</a>
         <a href="/admin/import.php" class="admin-nav__link <?= $currentPage === 'import.php' ? 'active' : '' ?>">Importer une liste de TFE</a>
         <a href="/admin/pages.php" class="admin-nav__link <?= in_array($currentPage, ['pages.php','pages-edit.php']) ? 'active' : '' ?>">Pages statiques</a>
+        <a href="/admin/tags.php" class="admin-nav__link <?= $currentPage === 'tags.php' ? 'active' : '' ?>">Mots-clés</a>
         <?php if ($thesisId && in_array($currentPage, ['edit.php', 'thanks.php'])): ?>
             <a href="/admin/edit.php?id=<?= intval($thesisId) ?>" class="admin-nav__link <?= $currentPage === 'edit.php' ? 'active' : '' ?>">Modifier</a>
         <?php endif; ?>