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/user.py

@@ -56,9 +56,8 @@ class User(Base, TimestampMixin):
     preferred_language = Column(String(5), nullable=True)
 
     # Relationships
-    marketplace_import_jobs = relationship(
-        "MarketplaceImportJob", back_populates="user"
-    )
+    # NOTE: marketplace_import_jobs relationship removed - owned by marketplace module
+    # Use: MarketplaceImportJob.query.filter_by(user_id=user.id) instead
     owned_companies = relationship("Company", back_populates="owner")
     vendor_memberships = relationship(
         "VendorUser", foreign_keys="[VendorUser.user_id]", back_populates="user"