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/vendor/__init__.py

@@ -20,6 +20,7 @@ from . import (
     content_pages,
     customers,
     dashboard,
+    email_templates,
     features,
     info,
     inventory,
@@ -59,6 +60,7 @@ router.include_router(auth.router, tags=["vendor-auth"])
 router.include_router(dashboard.router, tags=["vendor-dashboard"])
 router.include_router(profile.router, tags=["vendor-profile"])
 router.include_router(settings.router, tags=["vendor-settings"])
+router.include_router(email_templates.router, tags=["vendor-email-templates"])
 router.include_router(onboarding.router, tags=["vendor-onboarding"])
 
 # Business operations (with prefixes: /products/*, /orders/*, etc.)
@@ -83,11 +85,7 @@ router.include_router(features.router, tags=["vendor-features"])
 router.include_router(usage.router, tags=["vendor-usage"])
 
 # Content pages management
-router.include_router(
-    content_pages.router,
-    prefix="/{vendor_code}/content-pages",
-    tags=["vendor-content-pages"],
-)
+router.include_router(content_pages.router, tags=["vendor-content-pages"])
 
 # Vendor info endpoint - MUST BE LAST! Has catch-all GET /{vendor_code}
 router.include_router(info.router, tags=["vendor-info"])