Fix loose comparisons in authorization checks

- TicketModel.php: fix bind_param "sssi"→"issi" for ticketId in addComment()
- TicketModel.php: use strict (int) cast === for confidential ticket access check
- update_ticket.php: use strict (int) cast !== for creator/assignee auth check
- AttachmentModel.php: use strict (int) cast === for upload ownership check

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

models/AttachmentModel.php

@@ -131,7 +131,7 @@ class AttachmentModel {
         }
 
         $attachment = $this->getAttachment($attachmentId);
-        return $attachment && $attachment['uploaded_by'] == $userId;
+        return $attachment && (int)$attachment['uploaded_by'] === (int)$userId;
     }
 
     /**