style: apply black and isort formatting across entire codebase

- Standardize quote style (single to double quotes)
- Reorder and group imports alphabetically
- Fix line breaks and indentation for consistency
- Apply PEP 8 formatting standards

Also updated Makefile to exclude both venv and .venv from code quality checks.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/conftest.py

@@ -7,13 +7,13 @@ from sqlalchemy.pool import StaticPool
 
 from app.core.database import Base, get_db
 from main import app
+from models.database.inventory import Inventory
 # Import all models to ensure they're registered with Base metadata
 from models.database.marketplace_import_job import MarketplaceImportJob
 from models.database.marketplace_product import MarketplaceProduct
-from models.database.vendor import Vendor
 from models.database.product import Product
-from models.database.inventory import Inventory
 from models.database.user import User
+from models.database.vendor import Vendor
 
 # Use in-memory SQLite database for tests
 SQLALCHEMY_TEST_DATABASE_URL = "sqlite:///:memory:"