Resend integration: build email around identity and suppression

Reliable email needs verified domains, expected recipients, idempotent sends, and durable suppression handling.

• Separate transactional and marketing streams.
• Record consent or transactional purpose.
• Render and validate templates before sending.
• Verify webhooks and deduplicate events.
• Honor bounces, complaints, and unsubscribes across retries.

Test provider timeout after acceptance, attachment limits, international addresses, template changes, and account quotas. Keep recipient and body data out of routine logs. Track provider IDs so support can follow one message.