Add transactional email service for auth and certificates (Nodemailer + templates)

[Jayrodri088]

Summary

This PR implements a real backend email notification layer for ByteChain Academy and replaces the previous forgot-password stub behavior.

What was broken

The auth forgot-password flow generated a reset token but did not send an email, making password recovery non-functional in real deployments.

What this PR adds

• Introduces a dedicated EmailModule and EmailService powered by Nodemailer.
• Adds reusable HTML template builders for key transactional emails.
• Wires welcome email delivery into user registration.
• Wires password reset email delivery into forgot-password.
• Wires certificate email delivery into automatic course certificate issuance.
• Documents SMTP configuration in .env.example.
• Implements graceful degradation: when SMTP_HOST is missing, email content is logged instead of throwing, so app behavior remains stable in local/dev/test environments.

Scope of Changes

New files

• backend/src/email/email.module.ts
• backend/src/email/email.service.ts
• backend/src/email/templates/welcome.template.ts
• backend/src/email/templates/password-reset.template.ts
• backend/src/email/templates/certificate.template.ts
• backend/src/email/templates/streak-reminder.template.ts

Updated files

• backend/src/auth/auth.service.ts
• backend/src/auth/auth.module.ts
• backend/src/certificates/certificates.service.ts
• backend/src/certificates/certificates.module.ts
• backend/src/app.module.ts
• backend/.env.example
• backend/package.json
• backend/package-lock.json

Implementation Details

Email service design

• EmailService reads SMTP config from ConfigService:
  • SMTP_HOST
  • SMTP_PORT
  • SMTP_USER
  • SMTP_PASS
  • SMTP_FROM_NAME
  • SMTP_FROM_EMAIL
• If SMTP_HOST is present, service creates a Nodemailer transport and sends real emails.
• If SMTP_HOST is absent, service logs an email payload (to, subject, html) via Nest Logger.

Supported email methods

• sendWelcomeEmail(to, username)
• sendPasswordResetEmail(to, resetToken, resetUrl)
• sendCertificateEmail(to, username, courseName, certificateHash, downloadUrl)
• sendStreakReminderEmail(to, username, streak)

Template behavior

Each template returns { subject, html } and uses:

• Inline CSS for email-client compatibility
• ByteChain Academy branding
• Green-themed palette
• Mobile-safe table layout

Auth flow updates

• register() now sends a welcome email after successful user creation.
• forgotPassword() now:
  • Generates reset token
  • Builds a reset URL using CLIENT_URL fallback (http://localhost:3000)
  • Sends password reset email
  • No longer returns token in API response

Certificate flow updates

• issueCertificateForCourse() now sends certificate email after issuance, including:
  • learner name
  • course title
  • certificate hash
  • certificate URL (CLIENT_URL fallback-based)

Config

Added to .env.example:

• SMTP_HOST=
• SMTP_PORT=587
• SMTP_USER=
• SMTP_PASS=
• SMTP_FROM_NAME=ByteChain Academy
• [email protected]

Backward Compatibility / Safety

• No hard failure when SMTP is unset.
• Existing app flows continue working in dev/test with logging fallback.
• Keeps behavior production-ready once SMTP env is configured.

Validation / Test Plan

• Register a new user and verify welcome email is sent (or logged when SMTP not set)
• Trigger forgot-password and verify reset email includes reset link/token (or logged)
• Complete a course and verify certificate email includes certificate hash/link (or logged)
• Confirm app startup succeeds with no SMTP_HOST configured
• Confirm app startup and mail delivery succeed with valid SMTP credentials configured
• Run backend build and lint/test checks as part of CI

Closes: #207

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[portableDD]

@Jayrodri088 please resolve conflicts

[Jayrodri088]

@portableDD Conflicts resolved