feat: integrate Resend for transactional email delivery

Replace the stubbed email service with Resend API integration.
Emails are sent with branded HTML templates including activation,
welcome, payment failed, member invite, and password reset flows.

- Install resend@6.9.4 in backend
- Rewrite EmailService with Resend SDK + graceful fallback to
  stub mode when API key is not configured
- Add branded HTML email template with CTA buttons, preheader
  text, and fallback URL for all email types
- Add reply-to support (sales@hoaledgeriq.com in production)
- Track send status (sent/failed) in shared.email_log metadata
- Add RESEND_API_KEY, RESEND_FROM_ADDRESS, RESEND_REPLY_TO env
  vars to both docker-compose.yml and docker-compose.prod.yml
- Add sendPasswordResetEmail() method for future use

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/package.json

@@ -45,6 +45,7 @@
     "pg": "^8.13.1",
     "qrcode": "^1.5.4",
     "reflect-metadata": "^0.2.2",
+    "resend": "^6.9.4",
     "rxjs": "^7.8.1",
     "stripe": "^20.4.1",
     "typeorm": "^0.3.20",