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/payments/definition.py

@@ -38,8 +38,9 @@ payments_module = ModuleDefinition(
     code="payments",
     name="Payment Gateways",
     description=(
-        "Payment gateway integrations for Stripe, PayPal, and bank transfers. "
-        "Provides payment processing, refunds, and payment method management."
+        "Core payment gateway integrations for Stripe, PayPal, and bank transfers. "
+        "Provides payment processing, refunds, and payment method management. "
+        "Required by billing module for subscription payments."
     ),
     version="1.0.0",
     features=[
@@ -80,7 +81,7 @@ payments_module = ModuleDefinition(
             "payment-methods",  # Manage stored payment methods
         ],
     },
-    is_core=False,
+    is_core=True,  # Core module - required for billing and subscription management
     is_internal=False,
     # =========================================================================
     # Self-Contained Module Configuration