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>

controllers/DashboardController.php

@@ -186,6 +186,10 @@ class DashboardController {
         $categories = [];
         $types = [];
 
+        if (!$result) {
+            return ['categories' => $categories, 'types' => $types];
+        }
+
         while ($row = $result->fetch_assoc()) {
             if ($row['field'] === 'category' && !in_array($row['value'], $categories, true)) {
                 $categories[] = $row['value'];