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/bulk_operation.php

@@ -110,6 +110,10 @@ if (isset($result['error'])) {
         'error' => $result['error']
     ]);
 } else {
+    // Invalidate stats cache so dashboard tiles reflect changes immediately
+    require_once dirname(__DIR__) . '/models/StatsModel.php';
+    (new StatsModel($conn))->invalidateCache();
+
     $message = "Bulk operation completed: {$result['processed']} succeeded, {$result['failed']} failed";
     if ($inaccessibleCount > 0) {
         $message .= " ($inaccessibleCount skipped - no access)";