feat: implement email template system with vendor overrides

Add comprehensive email template management for both admin and vendors:

Admin Features:
- Email templates management page at /admin/email-templates
- Edit platform templates with language support (en, fr, de, lb)
- Preview templates with sample variables
- Send test emails
- View email logs per template

Vendor Features:
- Email templates customization page at /vendor/{code}/email-templates
- Override platform templates with vendor-specific versions
- Preview and test customized templates
- Revert to platform defaults

Technical Changes:
- Migration for vendor_email_templates table
- VendorEmailTemplate model with override management
- Enhanced EmailService with language resolution chain
  (customer preferred -> vendor preferred -> platform default)
- Branding resolution (Wizamart default, removed for whitelabel)
- Platform-only template protection (billing templates)
- Admin and vendor API endpoints with full CRUD
- Updated seed script with billing and team templates

Files: 22 changed, ~3,900 lines added

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

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

models/database/__init__.py

@@ -20,6 +20,7 @@ from .content_page import ContentPage
 from .customer import Customer, CustomerAddress
 from .password_reset_token import PasswordResetToken
 from .email import EmailCategory, EmailLog, EmailStatus, EmailTemplate
+from .vendor_email_template import VendorEmailTemplate
 from .feature import Feature, FeatureCategory, FeatureCode, FeatureUILocation
 from .inventory import Inventory
 from .inventory_transaction import InventoryTransaction, TransactionType
@@ -112,6 +113,7 @@ __all__ = [
     "EmailLog",
     "EmailStatus",
     "EmailTemplate",
+    "VendorEmailTemplate",
     # Features
     "Feature",
     "FeatureCategory",