security: fix all LOW priority items from TODO.SECURITY.md

Item 13 — Remove deprecated X-XSS-Protection header
- nginx/posterg.conf: header removed (was '1; mode=block')
- nginx/SECURITY_HEADERS.md: new file documenting header decisions
  and explaining why X-XSS-Protection is counterproductive

Item 14 — Add rel="noreferrer" to external target="_blank" link
- public/admin/thanks.php: rel="noopener" → rel="noopener noreferrer"

Item 15 — Explicit (int) casts on all integer HTML outputs
- public/index.php: (int) on item id, page numbers
- public/search.php: (int) on totalItems, year options, item id, pagination

Item 16 — Remove unused DATABASE_PATH constant
- config/bootstrap.php: define('DATABASE_PATH', ...) removed

docs/TODO.SECURITY.md updated: items 13-16 marked resolved and
moved to the ✅ Resolved section.

public/index.php

@@ -24,7 +24,7 @@ include APP_ROOT . '/includes/header.php';
 
 <main>
     <?php foreach ($itemsToLoad as $item): ?>
-        <a href="memoire.php?id=<?= $item["id"] ?>" class="card-link">
+        <a href="memoire.php?id=<?= (int)$item["id"] ?>" class="card-link">
             <div class="card">
                 <div class="card-content">
                     <h2 class="title"><?= htmlspecialchars($item["title"]) ?></h2>
@@ -43,14 +43,14 @@ include APP_ROOT . '/includes/header.php';
 <?php if ($totalPages > 1): ?>
 <nav class="pagination">
     <?php if ($page > 1): ?>
-        <a href="?page=<?= $page - 1 ?>" class="pagination-previous">Précédent</a>
+        <a href="?page=<?= (int)($page - 1) ?>" class="pagination-previous">Précédent</a>
     <?php endif; ?>
 
     <?php if ($page < $totalPages): ?>
-        <a href="?page=<?= $page + 1 ?>" class="pagination-next">Suivant</a>
+        <a href="?page=<?= (int)($page + 1) ?>" class="pagination-next">Suivant</a>
     <?php endif; ?>
 
-    <span class="pagination-info">Page <?= $page ?> sur <?= $totalPages ?></span>
+    <span class="pagination-info">Page <?= (int)$page ?> sur <?= (int)$totalPages ?></span>
 </nav>
 <?php endif; ?>