Enforce roles on admin endpoints and return real status codes

- users, SMTP and batch-lock endpoints are owner/admin only
- engines answer 400/403/404/409/500 instead of 200 with an error body;
  database errors no longer leak to the client

app/accounting/api/engine/get_journal_listing.php

@@ -11,8 +11,13 @@ try {
         trim((string)($data['date_to'] ?? ''))
     );
     $answer['success'] = 1;
+} catch (PDOException $e) {
+    error_log('[' . basename(__FILE__) . '] ' . $e->getMessage());
+    $answer['message'] = 'Database error, please try again.';
+    http_response_code(500);
 } catch (Exception $e) {
     $answer['message'] = $e->getMessage();
+    http_response_code(400);
 }
 
 exit(json_encode($answer));