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/routes/admin_pages.py

@@ -500,6 +500,30 @@ async def admin_notifications_page(
     )
 
 
+# ============================================================================
+# EMAIL TEMPLATES ROUTES
+# ============================================================================
+
+
+@router.get("/email-templates", response_class=HTMLResponse, include_in_schema=False)
+async def admin_email_templates_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render email templates management page.
+    Shows all platform email templates with edit capabilities.
+    """
+    return templates.TemplateResponse(
+        "admin/email-templates.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
 # ============================================================================
 # MESSAGING ROUTES
 # ============================================================================