feat: use gemini-2.0-flash-lite for highest free-tier RPD (1500/day)

api/chat.php

@@ -174,7 +174,8 @@ if (!function_exists('curl_init')) {
 }
 
 // Call Gemini API via cURL
-$url = "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash-preview-native-audio-dialog:generateContent?key=" . urlencode($apiKey);
+// gemini-2.0-flash-lite: highest free-tier quota (1,500 req/day)
+$url = "https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash-lite:generateContent?key=" . urlencode($apiKey);
 
 $ch = curl_init($url);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);