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/tenancy/models/__init__.py

@@ -14,12 +14,12 @@ This is the canonical location for tenancy module models including:
 # These imports are NOT re-exported, just ensure models are registered with SQLAlchemy
 # before the tenancy models are loaded.
 #
-# Relationship chain being resolved:
+# Relationship being resolved:
 # - Platform.admin_menu_configs -> "AdminMenuConfig" (in core module)
-# - User.marketplace_import_jobs -> "MarketplaceImportJob" (in marketplace module)
-# - Vendor.marketplace_import_jobs -> "MarketplaceImportJob" (in marketplace module)
+#
+# NOTE: MarketplaceImportJob relationships have been moved to the marketplace module.
+# Optional modules own their relationships to core models, not vice versa.
 from app.modules.core.models import AdminMenuConfig  # noqa: F401
-from app.modules.marketplace.models.marketplace_import_job import MarketplaceImportJob  # noqa: F401
 
 from app.modules.tenancy.models.admin import (
     AdminAuditLog,