Implement bulk_delete operation; validate operation types

- TicketModel: add deleteTicket() that removes all child records
  (comments, watchers, dependencies, attachments, custom fields)
  then deletes the ticket and cleans up physical attachment files
- BulkOperationsModel: add bulk_delete case to processBulkOperation()
  so the "Bulk Delete" UI button actually works instead of silently
  failing with N failures
- bulk_operation.php: validate operation_type against whitelist to
  reject unknown operations early with a proper error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models/BulkOperationsModel.php

@@ -173,6 +173,14 @@ class BulkOperationsModel {
                             }
                         }
                         break;
+
+                    case 'bulk_delete':
+                        $success = $ticketModel->deleteTicket($ticketId);
+                        if ($success) {
+                            $auditLogModel->log($operation['performed_by'], 'delete', 'ticket', $ticketId,
+                                ['bulk_operation_id' => $operationId]);
+                        }
+                        break;
                 }
 
                 if ($success) {