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/billing/definition.py

@@ -77,11 +77,12 @@ billing_module = ModuleDefinition(
     code="billing",
     name="Billing & Subscriptions",
     description=(
-        "Platform subscription management, vendor billing, and invoice history. "
+        "Core subscription management, tier limits, vendor billing, and invoice history. "
+        "Provides tier-based feature gating used throughout the platform. "
         "Uses the payments module for actual payment processing."
     ),
     version="1.0.0",
-    requires=["payments"],  # Depends on payments module for payment processing
+    requires=["payments"],  # Depends on payments module (also core) for payment processing
     features=[
         "subscription_management",  # Manage subscription tiers
         "billing_history",  # View invoices and payment history
@@ -200,7 +201,7 @@ billing_module = ModuleDefinition(
             ),
         ],
     },
-    is_core=False,  # Billing can be disabled (e.g., internal platforms)
+    is_core=True,  # Core module - tier limits and subscription management are fundamental
     # Context providers for dynamic page context
     context_providers={
         FrontendType.PLATFORM: _get_platform_context,