Fix bulk operation dropping tickets with leading-zero IDs, add query null-check

bulk_operation.php: ticket ID validation was converting IDs to int then back
to string, so '000123456' became '123456' which never matched the DB VARCHAR
key, silently rejecting ~11% of tickets from bulk operations. Now validates
with ctype_digit() to preserve leading zeros.

TicketModel::getTicketsByIds(): changed intval() to strval() and bind type
'i' to 's' so VARCHAR ticket_id columns are queried consistently as strings.

DashboardController::getCategoriesAndTypes(): added null check on query
result before calling fetch_assoc() to prevent TypeError if query fails.

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

api/bulk_operation.php

@@ -49,10 +49,10 @@ if (!$operationType || empty($ticketIds)) {
     exit;
 }
 
-// Validate ticket IDs are positive integers
+// Validate ticket IDs: must be non-empty numeric strings (allows leading zeros)
 $ticketIds = array_values(array_filter(array_map(function($id) {
-    $int = (int)$id;
-    return ($int > 0 && (string)$int === (string)$id) ? $int : null;
+    $s = trim((string)$id);
+    return (ctype_digit($s) && (int)$s > 0) ? $s : null;
 }, $ticketIds)));
 if (empty($ticketIds)) {
     echo json_encode(['success' => false, 'error' => 'No valid ticket IDs provided']);