Fix follow-up cron wiring

[MaxLinCode]

Summary

• add the Vercel cron schedule and accept GET requests for the follow-up cron route
• make follow-up bundle idempotency deterministic across repeated runner attempts
• update tests and deployment docs for the production cron contract

Verification

• pnpm --filter @atlas/web typecheck
• pnpm --filter @atlas/web test -- --run src/app/api/cron/send-followups/route.test.ts src/lib/server/follow-up.test.ts

[vercel]

Deployment failed with the following error:

Hobby accounts are limited to daily cron jobs. This cron expression (*/10 * * * *) would run more than once per day. Upgrade to the Pro plan to unlock all Cron Jobs features on Vercel.

Learn More: https://vercel.link/3Fpeeb1