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/update_ticket.php

@@ -270,7 +270,13 @@ try {
     
     // Discard any output that might have been generated
     ob_end_clean();
-    
+
+    // Invalidate stats cache on successful ticket update
+    if (!empty($result['success'])) {
+        require_once dirname(__DIR__) . '/models/StatsModel.php';
+        (new StatsModel($conn))->invalidateCache();
+    }
+
     // Return response
     if (!empty($result['http_status'])) {
         http_response_code($result['http_status']);