debug: add matchmaking debug info to response

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

api/matchmaking.php

@@ -127,7 +127,9 @@ function handleQueue($db, string $userId, array $input): void {
         }
     }
 
-    jsonResponse(['queued' => true]);
+    // Debug: return what the search found
+    $debugOpponents = is_array($opponents) ? count($opponents) : 'not_array';
+    jsonResponse(['queued' => true, 'debug_found' => $debugOpponents, 'debug_user' => $userId, 'debug_url' => $searchUrl ?? 'none']);
 }