feat: add resend service emulator

[mvanhorn]

Summary

Adds a Resend email API emulator with a web inbox UI for viewing sent emails locally. Emails, domains, API keys, audiences, and contacts are all stateful and persist in memory.

Why this matters

Every Next.js SaaS app sends transactional email (signup, password reset, receipts). Resend is the standard choice in the Vercel ecosystem - it's in vercel-labs/workflow-builder-template (1,074 stars) and has deep React Email integration.

Currently developers either use real API keys in dev (costs money, sends real emails) or skip email testing entirely. The only local tool is resend-local (11 stars) which mocks only the send endpoint. Resend's SDK supports RESEND_BASE_URL for drop-in emulator redirect.

Changes

New package: @internal/resend (~500 lines across 14 files)

Endpoints:

• POST /emails - send email, validate required fields, dispatch email.sent + email.delivered webhooks
• POST /emails/batch - send up to 100 emails
• GET /emails / GET /emails/:id - list and retrieve with Resend's cursor pagination format
• POST /emails/:id/cancel - cancel scheduled emails
• Domain CRUD with auto-generated DNS records and instant verify
• API key CRUD with re_ prefix tokens
• Audience + contact CRUD with webhook dispatch

Inbox UI at /inbox - the differentiator:

Uses only core CSS classes (zero inline styles). HTML email preview rendered in a sandboxed iframe.

Integration: All 7 points wired in start.ts + list.ts SERVICE_DESCRIPTIONS updated.

Testing

23 tests covering all endpoints, error format ({ statusCode, name, message }), batch send, cursor pagination, inbox UI rendering, and seed config.

Dogfooding

Ran the emulator, sent 3 test emails (welcome, invoice, password reset), verified inbox shows from/to/subject/status, clicked through to detail page with HTML preview. Screenshots above are from the running emulator.

This contribution was developed with AI assistance (Claude Code).

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[ctate]

Thanks for this excellent contribution! The Resend emulator follows existing service patterns really well, the inbox UI is a great differentiator, and all 23 tests pass cleanly. A few small things before merging:

1. Remove unused resendId() — helpers.ts:9-11 defines resendId() as an alias for generateUuid() but it's never called anywhere. Let's remove the dead code.

2. Type the as any casts — routes/emails.ts:137 and routes/domains.ts:93 use as any in update() calls, and formatEmail/formatDomain use (email: any) params. Would be good to use the proper entity types (ResendEmail, ResendDomain) instead.

None of these are blocking — happy to merge once addressed. Great work!

[mvanhorn]

Addressed in 2764090:

• Removed unused resendId() from helpers.ts
• Replaced both as any casts with properly typed Partial<T> objects (removed the unnecessary as const assertions that were causing the type mismatch)

[ctate]

Thanks for the updates @mvanhorn

[mvanhorn]

yay

[mvanhorn]

Thanks for merging both emulators!

[mvanhorn]

Thanks for the quick merge!

[mvanhorn]

Thanks for merging both, @ctate.