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/public/admin/actions/export-files.php

@@ -0,0 +1,48 @@
+<?php
+/**
+ * Export all thesis files as a ZIP archive.
+ *
+ * The ZIP contains:
+ *   - manifest.json   — maps every thesis to its files with DB-relevant metadata
+ *   - files/           — mirror of storage/theses/ directory structure
+ *
+ * Thin dispatcher — delegates all data assembly to ExportController,
+ * then streams the response.
+ */
+require_once __DIR__ . "/../../../bootstrap.php";
+require_once __DIR__ . '/../../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+require_once APP_ROOT . '/src/Controllers/ExportController.php';
+require_once APP_ROOT . '/src/AdminLogger.php';
+
+try {
+    $controller = ExportController::create();
+
+    // Build the zip
+    $zipPath = $controller->createExportZip();
+
+    $fileSize = filesize($zipPath);
+    $fileCount = count($controller->getAllThesisFiles());
+
+    // Audit log (before headers in case log fails — but it's best-effort anyway)
+    AdminLogger::make()->logFilesExport($fileCount, (int)$fileSize);
+
+    $filename = 'xamxam-files-' . date('Y-m-d') . '.zip';
+
+    header('Content-Type: application/zip');
+    header('Content-Disposition: attachment; filename="' . $filename . '"');
+    header('Content-Length: ' . $fileSize);
+    header('Cache-Control: no-cache, must-revalidate');
+
+    readfile($zipPath);
+
+    // Clean up temp file
+    @unlink($zipPath);
+} catch (Exception $e) {
+    error_log('Files export error: ' . $e->getMessage());
+    http_response_code(500);
+    exit('Erreur lors de la création de l\'archive : ' . htmlspecialchars($e->getMessage()));
+}
+
+exit;