feat: add email system with multi-provider support

Implements a comprehensive email system with:
- Multi-provider support (SMTP, SendGrid, Mailgun, Amazon SES)
- Database-stored templates with i18n (EN, FR, DE, LB)
- Jinja2 template rendering with variable interpolation
- Email logging for debugging and compliance
- Debug mode for development (logs instead of sending)
- Welcome email integration in signup flow

New files:
- models/database/email.py: EmailTemplate and EmailLog models
- app/services/email_service.py: Provider abstraction and service
- scripts/seed_email_templates.py: Template seeding script
- tests/unit/services/test_email_service.py: 28 unit tests
- docs/features/email-system.md: Complete documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -201,6 +201,7 @@ nav:
       - Platform Homepage: features/platform-homepage.md
       - Vendor Landing Pages: features/vendor-landing-pages.md
       - Subscription & Billing: features/subscription-billing.md
+      - Email System: features/email-system.md
 
     # --- User Guides ---
     - User Guides: