[codex] Fix ZeptoMail auth email sending

[rahuliitk]

Summary

• Replace the ZeptoMail SDK usage in the auth mailer with a direct REST fetch call.
• Derive the REST API endpoint from the existing SMTP_HOST=smtp.zeptomail.in value, while also supporting explicit ZEPTOMAIL_URL / ZEPTOMAIL_TOKEN env vars.
• Add focused mailer tests for endpoint derivation and controlled transport errors.

Why

The ECS task crash was caused by the ZeptoMail SDK throwing TypeError: resp.json is not a function after an auth email path attempted to send mail. The service itself recovered, but this made email-triggered requests capable of crashing the quickvoice-server container. Calling the ZeptoMail REST API directly gives us predictable error handling and avoids the SDK failure path.

Production env note

No mandatory AWS production env change is required for this PR if SMTP_HOST=smtp.zeptomail.in and SMTP_PASSWORD is already the ZeptoMail Send Mail/API token. Optional cleanup later: add explicit ZEPTOMAIL_URL=api.zeptomail.in and/or ZEPTOMAIL_TOKEN for clarity.

Validation

• node --import tsx --test apps/server/tests/*.test.ts apps/server/tests/**/*.test.ts