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/src/AdminLogger.php

@@ -185,6 +185,15 @@ class AdminLogger
         $this->write('system', 'db_export', 'success');
     }
 
+    /** Files export (ZIP with all thesis files + manifest) */
+    public function logFilesExport(int $fileCount, int $byteSize): void
+    {
+        $this->write('system', 'files_export', 'success', [
+            'file_count' => $fileCount,
+            'byte_size'  => $byteSize,
+        ]);
+    }
+
     /** Parametres: delete all TFEs */
     public function logDeleteAllTheses(int $count): void
     {