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

Items resolved:
- #3 (HIGH): Move file uploads outside webroot to STORAGE_ROOT (/var/www/posterg/storage).
  Uploads were previously stored in public/admin/actions/data/ which is web-accessible.
- #4 (HIGH): Align file paths and add media.php controller.
  DB paths are now storage-relative (theses/YEAR/ID/file, covers/file).
  New public/media.php serves files with path-traversal jail, MIME allow-list,
  and proper caching headers. memoire.php and search.php updated to use /media.php?path=.
  Also fixed: cover images were never recorded in thesis_files (broken INSERT).
- #5 (HIGH): RateLimit::getClientIdentifier() now uses REMOTE_ADDR only.
  HTTP_X_FORWARDED_FOR and HTTP_CLIENT_IP are attacker-controlled headers that
  allowed unlimited rate-limit bypass by rotating spoofed IPs.
- #6 (HIGH): Port public/admin/.htaccess security rules to nginx/posterg.conf.
  Apache .htaccess directives are silently ignored by nginx; none were active.
  CSP added to /admin/ location block, .log file denial added globally,
  autoindex off made explicit. Documented in nginx/HTACCESS_TO_NGINX.md.

Supporting changes:
- config/bootstrap.php: add STORAGE_ROOT constant
- nginx/SECURITY_HEADERS.md: updated to reflect admin CSP and pending public CSP
- docs/TODO.SECURITY.md: items #3-6 moved to resolved; priority order updated

public/search.php

@@ -341,9 +341,9 @@ include APP_ROOT . '/includes/header.php'; ?>
                   if (!empty($item['id'])) {
                     $files = $db->getThesisFiles($item['id']);
                     foreach ($files as $file) {
-                      $ext = strtolower(pathinfo($file['file_path'], PATHINFO_EXTENSION));
-                      if (in_array($ext, ['jpg', 'jpeg', 'png', 'gif']) && $file['file_type'] === 'main') {
-                        $coverImage = $file['file_path'];
+                      // Look for dedicated cover file (type = 'cover')
+                      if ($file['file_type'] === 'cover') {
+                        $coverImage = '/media.php?path=' . urlencode($file['file_path']);
                         break;
                       }
                     }