Fix intermittent scraper 504s and poisoned cache: single instance, 504 on timeout, 1h TTL

ecosystem.config.js

@@ -3,5 +3,12 @@ module.exports = [{
     name: 'prerender-app',
     cwd: '/home/node',
     exec_mode: 'cluster',
-    instances: process.env.PHANTOM_WORKERS || 3, // Adjust based on your CPU cores
+    // IMPORTANT: keep instances at 1. Every prerender instance spawns its own
+    // Chrome bound to debugging port 9222 (see server.js chromeFlags); with more
+    // than one instance they all fight over that port, all connect to whichever
+    // Chrome bound it first, and when that Chrome restarts every worker loses its
+    // websocket mid-request -> intermittent 504s for real crawlers (facebookexternalhit
+    // etc). One instance already serves N concurrent renders, one Chrome tab per
+    // request. See README "Reliability design".
+    instances: 1,
 }];