Fix ticket ID handling in assign and delete_attachment APIs

assign_ticket.php: preserve string ticket ID (ctype_digit validation)
  instead of (int) cast for consistent audit logging and URL generation.

delete_attachment.php: use string ticket_id from DB for the upload
  directory path — (int) cast was stripping leading zeros, causing
  the wrong path (/uploads/123456/) instead of /uploads/000123456/.
  Also pass raw string to getTicketById() to let TicketModel handle
  type coercion.

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

api/assign_ticket.php

@@ -14,14 +14,15 @@ if (!is_array($data)) {
     exit;
 }
 
-$ticketId = isset($data['ticket_id']) ? (int)$data['ticket_id'] : 0;
+$ticketIdRaw = isset($data['ticket_id']) ? trim((string)$data['ticket_id']) : '';
 $assignedTo = $data['assigned_to'] ?? null;
 
-if ($ticketId <= 0) {
+if (!ctype_digit($ticketIdRaw) || (int)$ticketIdRaw <= 0) {
     http_response_code(400);
     echo json_encode(['success' => false, 'error' => 'Ticket ID required']);
     exit;
 }
+$ticketId = $ticketIdRaw;
 
 $ticketModel = new TicketModel($conn);
 $auditLogModel = new AuditLogModel($conn);