Fix deleteTicket crash when ticket_custom_fields table doesn't exist

PHP 8.2 raises mysqli_sql_exception on prepare() for non-existent tables
rather than returning false. Wrap each child-table delete in try/catch and
silently skip tables that don't exist in all deployments, re-throwing for
unexpected errors.

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

models/TicketModel.php

@@ -747,6 +747,7 @@ class TicketModel {
         ];
 
         foreach ($children as $sql) {
+            try {
                 $stmt = $this->conn->prepare($sql);
                 if (!$stmt) continue;
                 // ticket_dependencies uses two placeholders
@@ -757,6 +758,12 @@ class TicketModel {
                 }
                 $stmt->execute();
                 $stmt->close();
+            } catch (mysqli_sql_exception $e) {
+                // Skip optional tables that may not exist in all deployments
+                if (strpos($e->getMessage(), "doesn't exist") === false) {
+                    throw $e;
+                }
+            }
         }
 
         $stmt = $this->conn->prepare("DELETE FROM tickets WHERE ticket_id = ?");