email-resend: pass the Resend Idempotency-Key HTTP header

[dorgz]

Summary

@payloadcms/email-resend has no way to send the Resend Idempotency-Key HTTP header. Please add one.

Facts (3.90.2)

• sendEmail in packages/email-resend/src/index.ts sends only the Authorization and Content-Type HTTP headers.
• message.headers goes into the JSON body as email headers. Resend reads the idempotency key from the HTTP header on POST /emails, not from an email header. Source: https://resend.com/docs/dashboard/emails/idempotency-keys
• Resend keeps a key for 24 hours. A second request with the same key and the same payload does not send the email again.

Why it matters

The Payload job queue claims a job with a read and then a write, with no row lock (updateJobs in @payloadcms/drizzle). Two runs that start at the same time can claim the same job. An email task then sends the email twice. On Vercel, where the queue runs from after() and a cron, overlap is possible. An idempotency key per job makes the send safe.

Proposal

One of these:

1. Read a reserved option from the message, for example sendEmail({ ..., idempotencyKey: 'job-123' }), and send it as the Idempotency-Key HTTP header.
2. Accept a headers hook in the adapter args that returns extra HTTP headers per message.

Option 1 is the smallest change. I can open a PR if the team agrees on the shape.