refactor: clean up legacy models and migrate remaining schemas

Delete empty stub files from models/database/:
- audit.py, backup.py, configuration.py, monitoring.py
- notification.py, payment.py, search.py, task.py

Delete re-export files:
- models/database/subscription.py → app.modules.billing.models
- models/database/architecture_scan.py → app.modules.dev_tools.models
- models/database/test_run.py → app.modules.dev_tools.models
- models/schema/subscription.py → app.modules.billing.schemas
- models/schema/marketplace.py (empty)
- models/schema/monitoring.py (empty)

Migrate schemas to canonical module locations:
- billing.py → app/modules/billing/schemas/
- vendor_product.py → app/modules/catalog/schemas/
- homepage_sections.py → app/modules/cms/schemas/

Keep as CORE (framework-level, used everywhere):
- models/schema/: admin, auth, base, company, email, image, media, team, vendor*
- models/database/: admin*, base, company, email, feature, media, platform*, user, vendor*

Update 30+ files to use canonical import locations.

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

tests/unit/services/test_billing_service.py

@@ -15,7 +15,7 @@ from app.services.billing_service import (
     SubscriptionNotCancelledError,
     TierNotFoundError,
 )
-from models.database.subscription import (
+from app.modules.billing.models import (
     AddOnProduct,
     BillingHistory,
     SubscriptionStatus,

tests/unit/services/test_capacity_forecast_service.py

@@ -20,7 +20,7 @@ from app.services.capacity_forecast_service import (
     CapacityForecastService,
     capacity_forecast_service,
 )
-from models.database.subscription import CapacitySnapshot
+from app.modules.billing.models import CapacitySnapshot
 
 
 @pytest.mark.unit

tests/unit/services/test_feature_service.py

@@ -6,7 +6,7 @@ import pytest
 from app.exceptions import FeatureNotFoundError, InvalidFeatureCodesError, TierNotFoundError
 from app.services.feature_service import FeatureService, feature_service
 from models.database.feature import Feature
-from models.database.subscription import SubscriptionTier, VendorSubscription
+from app.modules.billing.models import SubscriptionTier, VendorSubscription
 
 
 @pytest.mark.unit

tests/unit/services/test_stripe_webhook_handler.py

@@ -7,7 +7,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 from app.handlers.stripe_webhook import StripeWebhookHandler
-from models.database.subscription import (
+from app.modules.billing.models import (
     BillingHistory,
     StripeWebhookEvent,
     SubscriptionStatus,

tests/unit/services/test_usage_service.py

@@ -5,7 +5,7 @@ import pytest
 
 from app.services.usage_service import UsageService, usage_service
 from app.modules.catalog.models import Product
-from models.database.subscription import SubscriptionTier, VendorSubscription
+from app.modules.billing.models import SubscriptionTier, VendorSubscription
 from models.database.vendor import VendorUser