fix: replace Resend with Nodemailer SMTP

[Atanu2k4]

Summary

Removes the Resend SDK entirely and switches email delivery to Nodemailer via SMTP (Gmail).
Fixes OTP email failures on Railway caused by IPv6 routing issues on outbound SMTP connections.

Related issue

Fixes #60

Changes

• Removed resend package from dependencies (npm uninstall resend)
• Replaced Resend API calls in lib/notifyMail.ts with Nodemailer SMTP transport
• Added family: 4 to force IPv4 on Railway (fixes ENETUNREACH on IPv6 SMTP addresses)
• Added connection/greeting/socket timeouts for cloud environment stability
• Added tls: { rejectUnauthorized: false } to handle Railway TLS edge cases
• Rewrote lib/validateConfig.ts to validate SMTP_HOST, SMTP_USER, SMTP_PASS instead of RESEND_* env vars
• Deleted compiled lib/validateConfig.js (stale Resend artifact)
• Updated README.md — tech stack, prerequisites, and env var examples now reflect SMTP/Nodemailer

Testing

• Verified OTP email delivery works locally via Gmail SMTP on port 587
• Confirmed resend package is fully removed from node_modules and package.json
• Railway deployment uses SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASS env vars (no Resend keys needed)

Checklist

• Title follows the repository PR format
• The issue is linked with Fixes #60
• Testing notes are included

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Remove the Resend codes, files #60)

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Remove the Resend codes, files #60)

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Remove the Resend codes, files #60)

[github-actions]

✅ PR Validation Passed

Your PR title and description meet all requirements:

• Title follows conventional format
• All required sections are present
• Issue is properly linked (Remove the Resend codes, files #60)