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>

app/modules/marketplace/static/admin/js/marketplace-letzshop.js

@@ -701,7 +701,7 @@ function adminMarketplaceLetzshop() {
             this.successMessage = '';
 
             try {
-                const response = await apiClient.post(`/admin/vendors/${this.selectedVendor.id}/export/letzshop`, {
+                const response = await apiClient.post(`/admin/letzshop/vendors/${this.selectedVendor.id}/export`, {
                     include_inactive: this.exportIncludeInactive
                 });