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/messaging/definition.py

@@ -28,7 +28,10 @@ def _get_vendor_router():
 messaging_module = ModuleDefinition(
     code="messaging",
     name="Messaging & Notifications",
-    description="Internal messages, customer communication, and notifications.",
+    description=(
+        "Core email and notification system for user registration, password resets, "
+        "team invitations, and system notifications. Required for basic platform operations."
+    ),
     version="1.0.0",
     features=[
         "customer_messaging",  # Customer communication
@@ -157,7 +160,7 @@ messaging_module = ModuleDefinition(
             ),
         ],
     },
-    is_core=False,
+    is_core=True,  # Core module - email/notifications required for registration, password reset, etc.
     # =========================================================================
     # Self-Contained Module Configuration
     # =========================================================================