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/vendor.py

@@ -3,7 +3,9 @@
 Vendor model representing entities that sell products or services.
 
 This module defines the Vendor model along with its relationships to
-other models such as User (owner), Product, Customer, Order, and MarketplaceImportJob.
+other models such as User (owner), Product, Customer, and Order.
+
+Note: MarketplaceImportJob relationships are owned by the marketplace module.
 """
 
 import enum
@@ -143,9 +145,8 @@ class Vendor(Base, TimestampMixin):
     orders = relationship(
         "Order", back_populates="vendor"
     )  # Relationship with Order model for orders placed by this vendor
-    marketplace_import_jobs = relationship(
-        "MarketplaceImportJob", back_populates="vendor"
-    )  # Relationship with MarketplaceImportJob model for import jobs related to this vendor
+    # NOTE: marketplace_import_jobs relationship removed - owned by marketplace module
+    # Use: MarketplaceImportJob.query.filter_by(vendor_id=vendor.id) instead
 
     # Letzshop integration credentials (one-to-one)
     letzshop_credentials = relationship(