Add Open Graph and Twitter Card meta tags to all public pages

- templates/public/head.php: add centralised OG/Twitter tag rendering via $ogTags array;
  supports type, title, description, url, image, image_alt, site_name, article_author,
  article_published_time; twitter:card switches between summary_large_image / summary
  based on presence of og:image

- public/tfe.php: populate full article OG tags — og:type=article, canonical URL,
  og:image resolved from banner_path → first image file in thesis_files → omitted,
  og:image:alt, article:author, article:published_time (year-01-01); twitter:card
  summary_large_image when image present

- public/index.php, search.php, apropos.php, licence.php: add basic og:type=website
  tags (title, description, canonical url, site_name)

Sharing a thesis link on Slack, WhatsApp, iMessage, or any social platform will now
render a rich preview card with the thesis title, synopsis excerpt, and cover/banner image.

public/tfe.php

@@ -25,6 +25,37 @@ $metaDescription = mb_strlen($_synopsisRaw) > 160
     ? mb_substr($_synopsisRaw, 0, 157) . '…'
     : (empty($_synopsisRaw) ? 'Mémoire de fin d\'études – Posterg, répertoire des TFE de l\'erg.' : $_synopsisRaw);
 unset($_tfeAuthor, $_synopsisRaw);
+
+// --- Open Graph / Twitter Card tags ------------------------------------------
+$_ogBaseUrl = 'https://posterg.erg.be';
+// Resolve OG image: banner → first image file → none
+$_ogImage = '';
+if (!empty($data['banner_path'])) {
+    $_ogImage = $_ogBaseUrl . '/media.php?path=' . rawurlencode($data['banner_path']);
+} elseif (!empty($data['files'])) {
+    foreach ($data['files'] as $_f) {
+        $_ext = strtolower(pathinfo($_f['file_path'], PATHINFO_EXTENSION));
+        if (in_array($_ext, ['jpg', 'jpeg', 'png', 'gif', 'webp'])) {
+            $_ogImage = $_ogBaseUrl . '/media.php?path=' . rawurlencode($_f['file_path']);
+            break;
+        }
+    }
+    unset($_f, $_ext);
+}
+$ogTags = [
+    'type'                   => 'article',
+    'title'                  => $data['title'] . (!empty($data['authors']) ? ' – ' . $data['authors'] : ''),
+    'description'            => $metaDescription,
+    'url'                    => $_ogBaseUrl . '/tfe.php?id=' . $thesisId,
+    'image'                  => $_ogImage,
+    'image_alt'              => $data['title'] . (!empty($data['authors']) ? ' par ' . $data['authors'] : ''),
+    'site_name'              => 'Posterg – ERG',
+    'article_author'         => $data['authors'] ?? '',
+    'article_published_time' => !empty($data['year']) ? $data['year'] . '-01-01' : '',
+];
+unset($_ogBaseUrl, $_ogImage);
+// --- End Open Graph ----------------------------------------------------------
+
 $extraCss = ['assets/tfe.css'];
 ?>
 <?php include APP_ROOT . '/templates/public/head.php'; ?>