Domain, email DNS, and Resend transactional setup

[quinn-dougherty]

Description

Register the sending domain and wire up transactional email. PLAYBOOK.md §8.

Requirements

• Pick/register domain (`mathless.lean` is not a real TLD — use `mathless-lean.com` or similar).
• DNS: SPF, DKIM, DMARC records.
• Set up addresses: `hello@`, `coc@`.
• Resend via Vercel Marketplace (or Postmark fallback).

Templates to author

• Submission confirmation (+ edit link).
• Curator decision (accept/reject).
• Curator welcome (+ Clerk magic link).
• Submission decision (accept/reject).
• Payment receipt + single-use Discord invite.
• T-7 pre-event reminder.
• Post-event survey.

Dependencies

• Blocks receipts in Stripe webhook (Build /register + Stripe Checkout + webhook #4), curator accept flow (Build /curate form (curator application) #2), submission decisions (Build /submit form (papers / repos / breakout sessions) #1).