refactor: complete module-driven architecture migration

This commit completes the migration to a fully module-driven architecture:

## Models Migration
- Moved all domain models from models/database/ to their respective modules:
  - tenancy: User, Admin, Vendor, Company, Platform, VendorDomain, etc.
  - cms: MediaFile, VendorTheme
  - messaging: Email, VendorEmailSettings, VendorEmailTemplate
  - core: AdminMenuConfig
- models/database/ now only contains Base and TimestampMixin (infrastructure)

## Schemas Migration
- Moved all domain schemas from models/schema/ to their respective modules:
  - tenancy: company, vendor, admin, team, vendor_domain
  - cms: media, image, vendor_theme
  - messaging: email
- models/schema/ now only contains base.py and auth.py (infrastructure)

## Routes Migration
- Moved admin routes from app/api/v1/admin/ to modules:
  - menu_config.py -> core module
  - modules.py -> tenancy module
  - module_config.py -> tenancy module
- app/api/v1/admin/ now only aggregates auto-discovered module routes

## Menu System
- Implemented module-driven menu system with MenuDiscoveryService
- Extended FrontendType enum: PLATFORM, ADMIN, VENDOR, STOREFRONT
- Added MenuItemDefinition and MenuSectionDefinition dataclasses
- Each module now defines its own menu items in definition.py
- MenuService integrates with MenuDiscoveryService for template rendering

## Documentation
- Updated docs/architecture/models-structure.md
- Updated docs/architecture/menu-management.md
- Updated architecture validation rules for new exceptions

## Architecture Validation
- Updated MOD-019 rule to allow base.py in models/schema/
- Created core module exceptions.py and schemas/ directory
- All validation errors resolved (only warnings remain)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/api/v1/public/test_letzshop_vendors.py

@@ -6,14 +6,14 @@ Tests the /api/v1/public/letzshop-vendors/* endpoints.
 
 import pytest
 
-from models.database.vendor import Vendor
-from models.database.company import Company
+from app.modules.tenancy.models import Vendor
+from app.modules.tenancy.models import Company
 
 
 @pytest.fixture
 def test_owner_user(db, auth_manager):
     """Create a test owner user for the company."""
-    from models.database.user import User
+    from app.modules.tenancy.models import User
 
     user = User(
         email="owner@test.com",

tests/integration/api/v1/public/test_signup.py

@@ -8,10 +8,10 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
-from models.database.company import Company
+from app.modules.tenancy.models import Company
 from app.modules.billing.models import TierCode
-from models.database.user import User
-from models.database.vendor import Vendor
+from app.modules.tenancy.models import User
+from app.modules.tenancy.models import Vendor
 
 
 @pytest.fixture