feat: Add Resend as a transactional email provider

[babblebey]

Add support for Resend as a transactional email provider, alongside the existing ZeptoMail integration. Resend exposes a simple REST API for sending email and is a popular choice for developer-oriented apps.

Background

The email layer is already provider-agnostic behind a TransactionalEmailProvider interface (see src/server/email/). ZeptoMail is currently the only concrete provider. Resend should slot in as a second implementation following the same patterns.

Key files:

• src/server/email/types.ts — EmailDriver, TransactionalEmailProvider interface, shared types
• src/server/email/zeptomail-provider.ts — reference implementation to mirror
• src/server/email/provider.ts — factory/singleton with exhaustive driver switch
• src/env.js — env schema with ZeptoMail-conditional validation
• src/server/api/routers/setup.ts — setup health check that probes email credentials

Tasks

1. Update src/server/email/types.ts

Extend EmailDriver to include "resend":

// Before
export type EmailDriver = "zeptomail";

// After
export type EmailDriver = "zeptomail" | "resend";

Also update SendTransactionalEmailResult.driver — it already uses EmailDriver, so this should propagate automatically.

2. Add ResendEmailProvider

Create src/server/email/resend-provider.ts implementing the TransactionalEmailProvider interface, modelled after zeptomail-provider.ts:

export class ResendEmailProvider implements TransactionalEmailProvider {
  readonly driver = "resend" as const;

  private readonly apiKey: string;

  constructor() {
    this.apiKey = env.RESEND_API_KEY
      ? String(env.RESEND_API_KEY)
      : missingConfig("RESEND_API_KEY");
  }

  async send(message: TransactionalEmailMessage): Promise<SendTransactionalEmailResult> {
    // POST https://api.resend.com/emails
    // Authorization: Bearer <RESEND_API_KEY>
    // Body: { from, to, subject, html, text }
  }
}

The Resend API endpoint is https://api.resend.com/emails. Authentication uses a Bearer token. The request body maps cleanly from TransactionalEmailMessage:

Resend field	Source
from	message.from.email (with optional name as "Name <email>")
to	[message.to.email]
subject	message.subject
html	message.html
text	message.text

Include:

• Structured console.info / console.error logging matching the [tx-email:resend:*] pattern from the ZeptoMail implementation.
• Error mapping for 401/403 → authentication, 429 → rate-limit, 5xx → provider-unavailable, other 4xx → invalid-request.
• missingConfig() helper for required env var validation.

3. Update src/server/email/provider.ts

Wire ResendEmailProvider into the factory and driver resolver:

// createEmailProvider()
case "resend":
  return new ResendEmailProvider();

// getConfiguredEmailDriver()
case "resend":
  return "resend";

Update the error message to list resend as a valid option:

`Unsupported EMAIL_DRIVER value: ${rawDriver}. Expected one of: zeptomail, resend`

4. Update src/env.js

• Add "resend" to the EMAIL_DRIVER enum:

  EMAIL_DRIVER: z.enum(["zeptomail", "resend"]).optional(),

• Add a isResendDriver flag (mirroring isZeptoMailDriver).
• Add RESEND_API_KEY with a conditional .refine() that requires it when EMAIL_DRIVER=resend:

  RESEND_API_KEY: z.string().optional().refine(
    (value) => !isResendDriver || Boolean(value),
    { message: "RESEND_API_KEY is required when EMAIL_DRIVER=resend" }
  ),

• Add RESEND_API_KEY to runtimeEnv.

5. Update src/server/api/routers/setup.ts

Add a probeResendCredentials() function (analogous to probeZeptoMailCredentials()) that makes a lightweight request to https://api.resend.com/emails with a Bearer token to verify credentials, and wire it into the email health check block.

6. Export from src/server/email/index.ts

Export ResendEmailProvider from src/server/email/index.ts (or let it be picked up via the existing export * from "./provider" if only exposed through the factory).

7. Add tests

Add test/server/email/resend-provider.test.ts covering:

• Successful send maps response to SendTransactionalEmailResult correctly.
• 401/403 throws TransactionalEmailError with code: "authentication".
• 429 throws with code: "rate-limit" and retryable: true.
• 5xx throws with code: "provider-unavailable" and retryable: true.
• Network failure throws with code: "provider-unavailable".
• Missing RESEND_API_KEY throws with code: "configuration".

Update test/server/email/provider.test.ts to cover:

• createEmailProvider() returns a ResendEmailProvider when EMAIL_DRIVER=resend.
• getConfiguredEmailDriver() returns "resend" for that value.

8. Update docs

• Update README.md to document EMAIL_DRIVER=resend and RESEND_API_KEY.
• Update any other relevant docs (e.g. self-hosted setup guide) to list Resend as a supported email provider option alongside ZeptoMail.

Acceptance Criteria

• ResendEmailProvider implements the full TransactionalEmailProvider interface.
• EMAIL_DRIVER=resend + RESEND_API_KEY fully replaces ZeptoMail for invite and claim email delivery.
• Setup health check correctly probes and reports Resend credential validity.
• Existing ZeptoMail behaviour is completely unaffected.
• Tests pass for the new provider and updated factory.
• Docs are updated.