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>

app/api/v1/admin/__init__.py

@@ -33,6 +33,7 @@ from . import (
     content_pages,
     customers,
     dashboard,
+    email_templates,
     features,
     images,
     inventory,
@@ -163,6 +164,9 @@ router.include_router(notifications.router, tags=["admin-notifications"])
 # Include messaging endpoints
 router.include_router(messages.router, tags=["admin-messages"])
 
+# Include email templates management endpoints
+router.include_router(email_templates.router, tags=["admin-email-templates"])
+
 # Include log management endpoints
 router.include_router(logs.router, tags=["admin-logs"])