fix(infra): add explicit cron disable guard

[cola500]

• Adds DISABLE_CRONS=true early return to all Vercel cron routes.
• Prevents staging project from executing reminder/data-retention jobs.
• Adds production prebuild guard so DISABLE_CRONS=true cannot be deployed to production.
• Adds tests for cron skip behavior and prod-env guard.
• No Vercel/Supabase changes.
• No deploys.

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