Reduce CORS preflight cache time from 24h to 5min

Changed Access-Control-Max-Age from 86400s (24 hours) to 300s (5 minutes).
This allows faster recovery when CORS issues occur with preview deployments,
while still providing caching benefits to reduce OPTIONS requests.

Fixes issue where preview deployments couldn't access the worker proxy
unless in a private session due to cached CORS failures.

Author: claude

cloudflare-worker/worker.js

@@ -197,7 +197,7 @@ function handleCorsPreflight(request) {
       ...getCorsHeaders(request),
       'Access-Control-Allow-Methods': 'GET, OPTIONS',
       'Access-Control-Allow-Headers': 'Content-Type',
-      'Access-Control-Max-Age': '86400',
+      'Access-Control-Max-Age': '300', // 5 minutes - allows quick recovery from CORS issues
     },
   });
 }