refactor: move letzshop endpoints to marketplace module and add vendor service tests

Move letzshop-related functionality from tenancy to marketplace module:
- Move admin letzshop routes to marketplace/routes/api/admin_letzshop.py
- Move letzshop schemas to marketplace/schemas/letzshop.py
- Remove letzshop code from tenancy module (admin_vendors, vendor_service)
- Update model exports and imports

Add comprehensive unit tests for vendor services:
- test_company_service.py: Company management operations
- test_platform_service.py: Platform management operations
- test_vendor_domain_service.py: Vendor domain operations
- test_vendor_team_service.py: Vendor team management

Update module definitions:
- billing, messaging, payments: Minor definition updates

Add architecture proposals documentation:
- Module dependency redesign session notes
- Decouple modules implementation plan
- Module decoupling proposal

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

docs/guides/letzshop-admin-management.md

@@ -224,8 +224,8 @@ Configure Letzshop API access:
 |----------|--------|-------------|
 | `/admin/products` | GET | List marketplace products with filters |
 | `/admin/products/stats` | GET | Get product statistics |
-| `/admin/vendors/{id}/export/letzshop` | GET | Download CSV export |
-| `/admin/vendors/{id}/export/letzshop` | POST | Export to pickup folder |
+| `/admin/letzshop/vendors/{id}/export` | GET | Download CSV export |
+| `/admin/letzshop/vendors/{id}/export` | POST | Export to pickup folder |
 
 ### Jobs