fix(backend): wire email delivery for verification and notifications …

[kenzy-hue]

…(#528)

• Add EmailRateLimiterService: in-memory sliding window, max 3 emails/hour/user
• EmailService: add EMAIL_ENABLED dry-run mode (logs instead of sending when false), improve SMTP error logging with structured error message and stack trace
• EmailSender (notifications channel): wrap all email templates in branded HTML (Vaultix gradient header, CTA button, unsubscribe footer); add plain-text footer
• AuthService.sendEmailVerification:
  • Inject EmailRateLimiterService and enforce rate limit (TooManyRequestsException)
  • Check user's email notification preference and skip if opted out
  • Use branded HTML template with verification link, 24h expiry warning, greeting personalisation, and unsubscribe footer
  • Catch SMTP errors and log them — never crash the HTTP request
  • Save verification token before attempting send so outbox can retry
• AuthModule: provide EmailRateLimiterService
• EmailModule: export EmailRateLimiterService
• Add 18 unit tests in auth.service.spec.ts covering: basic send, rate limiting (per-user isolation), preference opt-out, SMTP crash isolation, template content

Closes #528

[drips-wave]

@kenzy-hue 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

[KuchiMercy]

@kenzy-hue Kindly resolve errors