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/integration/api/v1/platform/test_pricing.py

@@ -6,7 +6,7 @@ Tests the /api/v1/platform/pricing/* endpoints.
 
 import pytest
 
-from models.database.subscription import (
+from app.modules.billing.models import (
     AddOnProduct,
     SubscriptionTier,
     TierCode,

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

@@ -9,7 +9,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 from models.database.company import Company
-from models.database.subscription import TierCode
+from app.modules.billing.models import TierCode
 from models.database.user import User
 from models.database.vendor import Vendor