add file export system for admins

- ExportController: getAllThesisFiles(), buildExportManifest(), createExportZip()
  builds a ZIP archive with manifest.json + files/ mirror of storage/theses/
- Database: getAllThesisFilesForExport() queries all thesis_files + identifier
- AdminLogger: logFilesExport() audit log entry
- admin/actions/export-files.php: thin dispatcher, streams zip with headers
- templates/admin/index.php: 'Exporter fichiers' button next to CSV export

app/templates/admin/index.php

@@ -134,6 +134,9 @@ document.addEventListener('DOMContentLoaded', () => {
             <a href="/admin/actions/export-csv.php" class="btn btn--primary btn--sm">
                 Exporter CSV
             </a>
+            <a href="/admin/actions/export-files.php" class="btn btn--primary btn--sm">
+                Exporter fichiers
+            </a>
         </div>
     </div>