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/testing/admin-frontend-features.md

@@ -112,8 +112,8 @@
 | Verify vendor | `/api/v1/admin/vendors/{vendor_identifier}/verification` | PUT | Working |
 | Toggle vendor status | `/api/v1/admin/vendors/{vendor_identifier}/status` | PUT | Working |
 | Delete vendor (confirm) | `/api/v1/admin/vendors/{vendor_identifier}?confirm=true` | DELETE | Working |
-| Export Letzshop CSV (download) | `/api/v1/admin/vendors/{vendor_identifier}/export/letzshop` | GET | Working |
-| Export Letzshop CSV (to folder) | `/api/v1/admin/vendors/{vendor_identifier}/export/letzshop` | POST | Working |
+| Export Letzshop CSV (download) | `/api/v1/admin/letzshop/vendors/{vendor_id}/export` | GET | Working |
+| Export Letzshop CSV (to folder) | `/api/v1/admin/letzshop/vendors/{vendor_id}/export` | POST | Working |
 
 **Create Vendor Form Fields:**
 - [ ] name (required)