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>

.gitignore

@@ -5,6 +5,5 @@ node_modules/
 .env.local
 *.log
 storage/
-worker/worker.pid
 # Added by code-review-graph
 .code-review-graph/