Invalidate stats cache after any ticket-modifying API call

StatsModel::invalidateCache() was never called from any API, so the
60s cached stats persisted after bulk assign/status/priority changes,
ticket updates, assignments, and clones. Dashboard tiles showed stale
counts until the TTL expired.

Added invalidation to the four APIs that affect dashboard stat tiles:
- bulk_operation.php: after successful bulk assign/status/priority
- assign_ticket.php: after successful reassignment
- update_ticket.php: after any successful ticket update
- clone_ticket.php: after successful clone (open_tickets changes)

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

api/clone_ticket.php

@@ -111,6 +111,8 @@ try {
         $dependencyModel = new DependencyModel($conn);
         $dependencyModel->addDependency($result['ticket_id'], $sourceTicketId, 'relates_to', $userId);
 
+        require_once dirname(__DIR__) . '/models/StatsModel.php';
+        (new StatsModel($conn))->invalidateCache();
         echo json_encode([
             'success' => true,
             'new_ticket_id' => $result['ticket_id'],