refactor: migrate Feature to billing module and split ProductMedia to catalog

- Move Feature model from models/database/ to app/modules/billing/models/
  (tightly coupled to SubscriptionTier for tier-based access control)
- Move ProductMedia from models/database/media.py to app/modules/catalog/models/
  (product-specific media associations belong with catalog)
- Keep MediaFile as CORE in models/database/media.py (cross-cutting file storage)
- Convert legacy feature.py to re-export for backwards compatibility
- Update all imports to use canonical module locations

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

app/core/feature_gate.py

@@ -38,7 +38,7 @@ from sqlalchemy.orm import Session
 from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
 from app.services.feature_service import feature_service
-from models.database.feature import FeatureCode
+from app.modules.billing.models import FeatureCode
 from models.database.user import User
 
 logger = logging.getLogger(__name__)