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/Database.php

@@ -2188,6 +2188,21 @@ class Database
         ')->fetchAll();
     }
 
+    /**
+     * All thesis files for the file-export ZIP.
+     * Includes every thesis_files column + the thesis identifier for manifest
+     * construction.
+     */
+    public function getAllThesisFilesForExport(): array
+    {
+        return $this->pdo->query('
+            SELECT tf.*, t.identifier
+            FROM thesis_files tf
+            JOIN theses t ON t.id = tf.thesis_id
+            ORDER BY t.year DESC, t.title ASC, tf.sort_order ASC
+        ')->fetchAll();
+    }
+
     // ========================================================================
     // SINGLETON PATTERN ENFORCEMENT
     // ========================================================================