Enforce ticket visibility on attachment and update endpoints

- delete_attachment.php: check canUserAccessTicket() before allowing deletion; return 404 (not 403) for inaccessible tickets to prevent existence leakage
- upload_attachment.php: verify ticket access on both GET (list) and POST (upload) before processing
- update_ticket.php: pass currentUser to controller; add canUserAccessTicket() check before permission check; return 404 for inaccessible tickets instead of leaking existence via 403

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

api/delete_attachment.php

@@ -23,6 +23,7 @@ require_once dirname(__DIR__) . '/helpers/Database.php';
 require_once dirname(__DIR__) . '/helpers/ResponseHelper.php';
 require_once dirname(__DIR__) . '/models/AttachmentModel.php';
 require_once dirname(__DIR__) . '/models/AuditLogModel.php';
+require_once dirname(__DIR__) . '/models/TicketModel.php';
 require_once dirname(__DIR__) . '/middleware/CsrfMiddleware.php';
 
 header('Content-Type: application/json');
@@ -66,7 +67,14 @@ try {
         ResponseHelper::notFound('Attachment not found');
     }
 
-    // Check permission
+    // Verify user can access the parent ticket
+    $ticketModel = new TicketModel(Database::getConnection());
+    $ticket = $ticketModel->getTicketById((int)$attachment['ticket_id']);
+    if (!$ticket || !$ticketModel->canUserAccessTicket($ticket, $_SESSION['user'])) {
+        ResponseHelper::notFound('Attachment not found');
+    }
+
+    // Check permission (must be uploader or admin)
     $isAdmin = $_SESSION['user']['is_admin'] ?? false;
     if (!$attachmentModel->canUserDelete($attachmentId, $_SESSION['user']['user_id'], $isAdmin)) {
         ResponseHelper::forbidden('You do not have permission to delete this attachment');