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/search.php

@@ -160,8 +160,8 @@ include APP_ROOT . '/includes/header.php'; ?>
                   <select name="year">
                     <option value="">Toutes les années</option>
                     <?php foreach ($years as $year): ?>
-                      <option value="<?= $year; ?>" <?= (isset($_GET['year']) && $_GET['year'] == $year) ? 'selected' : ''; ?>>
-                        <?= $year; ?>
+                      <option value="<?= (int)$year; ?>" <?= (isset($_GET['year']) && $_GET['year'] == $year) ? 'selected' : ''; ?>>
+                        <?= (int)$year; ?>
                       </option>
                     <?php endforeach; ?>
                   </select>
@@ -326,14 +326,14 @@ include APP_ROOT . '/includes/header.php'; ?>
     <!-- Search results -->
     <?php if (!empty($searchParams)): ?>
       <div class="notification is-info is-light">
-        <strong><?= $totalItems; ?></strong> résultat<?= $totalItems > 1 ? 's' : ''; ?> trouvé<?= $totalItems > 1 ? 's' : ''; ?>
+        <strong><?= (int)$totalItems; ?></strong> résultat<?= $totalItems > 1 ? 's' : ''; ?> trouvé<?= $totalItems > 1 ? 's' : ''; ?>
       </div>
 
       <?php if (count($results) > 0): ?>
         <div class="columns is-multiline">
           <?php foreach ($results as $item): ?>
             <div class="column is-one-fifth">
-              <a href="memoire.php?id=<?= $item['id']; ?>" class="card-link">
+              <a href="memoire.php?id=<?= (int)$item['id']; ?>" class="card-link">
                 <div class="card">
                   <?php
                   // Get cover image from thesis_files if available
@@ -397,7 +397,7 @@ include APP_ROOT . '/includes/header.php'; ?>
                 <li>
                   <a href="?<?= http_build_query(array_merge($_GET, ['page' => $i])); ?>"
                      class="pagination-link <?= $i === $page ? 'is-current' : ''; ?>">
-                    <?= $i; ?>
+                    <?= (int)$i; ?>
                   </a>
                 </li>
               <?php endfor; ?>