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/models/__init__.py

@@ -18,9 +18,16 @@ Usage:
 # These imports are NOT re-exported, just ensure models are registered with SQLAlchemy
 # before the marketplace models are loaded.
 #
-# Relationship being resolved:
+# Relationships being resolved:
 # - LetzshopFulfillmentQueue.order -> "Order" (in orders module)
+# - MarketplaceImportJob.vendor -> "Vendor" (in tenancy module)
+# - MarketplaceImportJob.user -> "User" (in tenancy module)
+#
+# NOTE: This module owns the relationships to tenancy models (User, Vendor).
+# Core models should NOT have back-references to optional module models.
 from app.modules.orders.models import Order  # noqa: F401
+from app.modules.tenancy.models.user import User  # noqa: F401
+from app.modules.tenancy.models.vendor import Vendor  # noqa: F401
 
 from app.modules.marketplace.models.marketplace_product import (
     MarketplaceProduct,

app/modules/marketplace/models/marketplace_import_job.py

@@ -95,7 +95,9 @@ class MarketplaceImportJob(Base, TimestampMixin):
     completed_at = Column(DateTime(timezone=True))
 
     # Relationships
-    vendor = relationship("Vendor", back_populates="marketplace_import_jobs")
+    # NOTE: No back_populates - optional modules own relationships to core models
+    # Core models (User, Vendor) don't have back-references to optional modules
+    vendor = relationship("Vendor")
     user = relationship("User", foreign_keys=[user_id])
     errors = relationship(
         "MarketplaceImportError",