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>

models/TicketModel.php

@@ -558,8 +558,8 @@ class TicketModel {
             return [];
         }
 
-        // Sanitize ticket IDs
-        $ticketIds = array_map('intval', $ticketIds);
+        // Sanitize ticket IDs: cast to string to preserve leading zeros
+        $ticketIds = array_map('strval', $ticketIds);
 
         // Create placeholders for IN clause
         $placeholders = str_repeat('?,', count($ticketIds) - 1) . '?';
@@ -578,7 +578,7 @@ class TicketModel {
                 WHERE t.ticket_id IN ($placeholders)";
 
         $stmt = $this->conn->prepare($sql);
-        $types = str_repeat('i', count($ticketIds));
+        $types = str_repeat('s', count($ticketIds));
         $stmt->bind_param($types, ...$ticketIds);
         $stmt->execute();
         $result = $stmt->get_result();