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/assign_ticket.php

@@ -84,5 +84,7 @@ if (!$success) {
     http_response_code(500);
     apiRespond(['success' => false, 'error' => 'Failed to update ticket assignment']);
 } else {
+    require_once dirname(__DIR__) . '/models/StatsModel.php';
+    (new StatsModel($conn))->invalidateCache();
     apiRespond(['success' => true]);
 }