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

@@ -747,16 +747,23 @@ class TicketModel {
         ];
 
         foreach ($children as $sql) {
-            $stmt = $this->conn->prepare($sql);
-            if (!$stmt) continue;
-            // ticket_dependencies uses two placeholders
-            if (strpos($sql, 'depends_on_id') !== false) {
-                $stmt->bind_param('ss', $ticketId, $ticketId);
-            } else {
-                $stmt->bind_param('s', $ticketId);
+            try {
+                $stmt = $this->conn->prepare($sql);
+                if (!$stmt) continue;
+                // ticket_dependencies uses two placeholders
+                if (strpos($sql, 'depends_on_id') !== false) {
+                    $stmt->bind_param('ss', $ticketId, $ticketId);
+                } else {
+                    $stmt->bind_param('s', $ticketId);
+                }
+                $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->execute();
-            $stmt->close();
         }
 
         $stmt = $this->conn->prepare("DELETE FROM tickets WHERE ticket_id = ?");