feat: supervise worker with pm2 instead of cron PID-check

Passenger only manages the Next.js app process; the worker now gets
the same crash-recovery/auto-restart via pm2, replacing the cron-driven
PID-file restart hack. Updates README deployment steps accordingly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ecosystem.config.cjs

@@ -0,0 +1,16 @@
+module.exports = {
+  apps: [
+    {
+      name: 'ombrora-worker',
+      script: 'npm',
+      args: 'run worker',
+      cwd: __dirname,
+      autorestart: true,
+      max_restarts: 10,
+      restart_delay: 5000,
+      env: {
+        NODE_ENV: 'production',
+      },
+    },
+  ],
+}