fix: add shared Nodemailer SMTP service for backend email delivery

[wheval]

Summary

Adds a shared Nodemailer SMTP transport for backend email delivery and wires existing email flows through it.

Purpose / Motivation

Issue #364 is part of the testnet readiness wave. The backend previously created ad-hoc Nodemailer transports and logged alert emails instead of sending them. This change centralizes SMTP configuration so password resets, notifications, and hot-wallet alerts can use the same transport.

Changes Made

• Added backend/src/lib/smtp.service.ts as the shared Nodemailer transport with safe logging when SMTP is not configured
• Refactored admin-email.service.ts to use the shared SMTP service
• Added SmtpEmailProvider and auto-selects it when SMTP env vars are present
• Wired hot-wallet monitor email alerts through the shared SMTP service
• Documented SMTP env vars in .env.example and backend README
• Added unit tests for the SMTP service
• Removed duplicate env schema keys that blocked TypeScript compilation

How to Test

1. Without SMTP configured, start the backend and trigger an admin password reset request.
   • Expected: no email is sent; a development log entry is written with the reset URL.
2. Configure SMTP env vars (SMTP_HOST, SMTP_PORT, SMTP_FROM, optional auth) and restart the backend.
3. Trigger another admin password reset request.
   • Expected: reset email is delivered through your SMTP provider.
4. Trigger a hot-wallet low-balance alert with ALERT_EMAIL_RECIPIENTS configured.
   • Expected: alert email is sent when SMTP is configured, otherwise logged locally.
5. Run backend tests:

   cd backend && npm test -- src/lib/smtp.service.test.ts

Breaking Changes

• None.

Related Issues

Closes #364

Checklist

• Code builds successfully
• Tests added/updated
• No console errors
• Documentation updated (if needed)

[drips-wave]

@wheval Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Ceejaytech25]

LGTM!