Fix visibility enforcement and register missing API routes

Security fixes:
- add_comment.php: verify canUserAccessTicket() before allowing comment creation
- assign_ticket.php: use canUserAccessTicket() to prevent info leakage via 403 vs 404
- check_duplicates.php: apply getVisibilityFilter() so confidential ticket titles are not exposed in duplicate search results
- ticket_dependencies.php: verify ticket access on GET before returning dependency data

Route registration:
- Register 7 previously missing API endpoints in index.php: custom_fields, saved_filters, audit_log, user_preferences, download_attachment, clone_ticket, health

Frontend:
- ticket.js: fill empty catch block and empty else block in addComment() with proper error toasts

Documentation:
- README.md: document all API endpoints and update project structure listing

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

api/add_comment.php

@@ -28,6 +28,7 @@ try {
     require_once $commentModelPath;
     require_once $auditLogModelPath;
     require_once dirname(__DIR__) . '/helpers/Database.php';
+    require_once dirname(__DIR__) . '/models/TicketModel.php';
 
     // Check authentication via session
     if (session_status() === PHP_SESSION_NONE) {
@@ -71,6 +72,24 @@ try {
         exit;
     }
 
+    // Verify user can access the ticket before allowing a comment
+    $ticketModel = new TicketModel($conn);
+    $ticket = $ticketModel->getTicketById($ticketId);
+    if (!$ticket) {
+        http_response_code(404);
+        ob_end_clean();
+        header('Content-Type: application/json');
+        echo json_encode(['success' => false, 'error' => 'Ticket not found']);
+        exit;
+    }
+    if (!$ticketModel->canUserAccessTicket($ticket, $currentUser)) {
+        http_response_code(403);
+        ob_end_clean();
+        header('Content-Type: application/json');
+        echo json_encode(['success' => false, 'error' => 'Access denied']);
+        exit;
+    }
+
     // Initialize models
     $commentModel = new CommentModel($conn);
     $auditLog = new AuditLogModel($conn);