fix: repair Gemini response parsing and update model

Author: CodeWithSyed786

api/health.js

@@ -2,6 +2,6 @@ export default function handler(_req, res) {
   res.status(200).json({
     ok: true,
     aiConfigured: Boolean(process.env.GEMINI_API_KEY),
-    model: process.env.GEMINI_MODEL || "gemini-2.5-flash"
+    model: process.env.GEMINI_MODEL || "gemini-3.6-flash"
   });
 }