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.

public/apropos.php

@@ -25,6 +25,7 @@ $pd = new Parsedown();
 $pd->setSafeMode(true);
 $aboutHtml = $pd->text($rawContent);
 $pageTitle = 'À Propos – Posterg';
+$metaDescription = 'À propos de Posterg, le répertoire des mémoires de fin d\'études de l\'erg – École de Recherches Graphiques de Bruxelles.';
 $extraCss = ['assets/apropos.css'];
 ?>
 <?php include APP_ROOT . '/templates/public/head.php'; ?>