Add <meta name=description> to all public pages; improve page titles

- templates/public/head.php: emit <meta name="description"> when $metaDescription is set
- index.php: title → 'Posterg – Mémoires de l\'ERG'; description = site blurb
- tfe.php: title → '[Titre] – [Auteur] – Posterg'; description = synopsis excerpt (strip_tags, truncate 160)
- search.php: description = répertoire purpose blurb
- apropos.php: description = about-page blurb
- licence.php: description = licences blurb

Fixes WCAG 2.4.2 (Page Titled) for index.php and tfe.php.
All descriptions properly htmlspecialchars-escaped at render time.

templates/public/head.php

@@ -4,6 +4,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title><?= htmlspecialchars($pageTitle ?? 'Posterg') ?></title>
+    <?php if (!empty($metaDescription)): ?>
+    <meta name="description" content="<?= htmlspecialchars($metaDescription) ?>">
+    <?php endif; ?>
     <link rel="icon" type="image/svg+xml" href="/assets/admin_favicon.svg">
     <link rel="stylesheet" href="assets/modern-normalize.min.css">
     <link rel="stylesheet" href="assets/common.css">