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>

models/database/__init__.py

@@ -18,6 +18,7 @@ from .base import Base
 from .company import Company
 from .content_page import ContentPage
 from .customer import Customer, CustomerAddress
+from .email import EmailCategory, EmailLog, EmailStatus, EmailTemplate
 from .inventory import Inventory
 from .invoice import (
     Invoice,
@@ -101,6 +102,11 @@ __all__ = [
     # Customer
     "Customer",
     "CustomerAddress",
+    # Email
+    "EmailCategory",
+    "EmailLog",
+    "EmailStatus",
+    "EmailTemplate",
     # Product - Enums
     "ProductType",
     "DigitalDeliveryMethod",