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,

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"

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(