Add admin server status page

New page /admin/status.php gives a real-time health dashboard:

- Services panel: nginx (systemctl), php-fpm (auto-detects versioned unit names),
  site HTTP ping (curl HEAD with latency), SQLite DB (exists/writable/row count/size),
  storage directory (writable, banner/cover file counts), maintenance-mode flag.
- PHP runtime panel: version, SAPI, memory_limit, upload_max_filesize, post_max_size,
  max_execution_time.
- Disk usage bar for the partition containing APP_ROOT (colour-coded: green/amber/red).
- All shell calls go through safeExec() which suppresses stderr and checks exit code;
  systemctl/curl unavailability degrades gracefully to 'unknown' without fatal errors.
- 'Statut' nav link added to templates/admin/head.php (active state on status.php).

templates/admin/head.php

@@ -29,6 +29,7 @@
         <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>
+        <a href="/admin/status.php" class="admin-nav__link <?= $currentPage === 'status.php' ? 'active' : '' ?>">Statut</a>
         <?php if ($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; ?>