Fix two backend correctness issues

- Wrap setThesisJury() in a transaction: the method did a DELETE then multiple
  INSERTs with no atomicity guarantee. A partial failure (e.g. findOrCreateSupervisor
  throwing) would leave the jury table with orphaned rows. The fix uses
  pdo->inTransaction() to avoid nesting when called from within an outer transaction,
  and performs beginTransaction/commit/rollBack otherwise.

- Replace raw PDO query in admin/thanks.php with db->getThesisFiles(): the file
  listing after TFE submission was manually preparing a SELECT on thesis_files
  instead of calling the existing Database::getThesisFiles() method. Removes the
  getPDO() call entirely from that file.

public/admin/thanks.php

@@ -25,7 +25,6 @@ if (isset($_GET['id'])) {
     if ($thesisId !== false && $thesisId > 0) {
         try {
             $db = new Database();
-            $pdo = $db->getPDO();
 
             // Get thesis data
             $thesis = $db->getThesis($thesisId);
@@ -33,15 +32,7 @@ if (isset($_GET['id'])) {
             if (!$thesis) {
                 $error = "TFE non trouvé.";
             } else {
-                // Get associated files
-                $stmt = $pdo->prepare("
-                    SELECT file_type, file_name, file_size, mime_type, uploaded_at
-                    FROM thesis_files
-                    WHERE thesis_id = ?
-                    ORDER BY file_type, uploaded_at
-                ");
-                $stmt->execute([$thesisId]);
-                $files = $stmt->fetchAll();
+                $files = $db->getThesisFiles($thesisId);
             }
         } catch (Exception $e) {
             error_log("Error loading thesis: " . $e->getMessage());